Remove NULL checks for AFS_NONNULL parameters

Remove NULL checks for AFS_NONNULL parameters

Recent GCC warns about opr_Assert(p != NULL), where p is an
__attribute__((__nonnull__)) parameter, just like clang did before those
clang warnings were silenced by 11852, 11853.

Now, we could go and add more autoconf tests and pragmas to silence the
GCC versions of these warnings.  However, I maintain that silencing the
warnings is the wrong approach.  The asserts in question have no
purpose.  They do not add any safety, because GCC and clang are
optimizing them away at compile time (without proof!—they take the
declaration at its word that NULL will never be passed).  Just remove
them.

Fixes these warnings (errors with --enable-checking) from GCC 6.2:

In file included from casestrcpy.c:17:0:
casestrcpy.c: In function ‘opr_lcstring’:
casestrcpy.c:26:31: error: nonnull argument ‘d’ compared to NULL [-Werror=nonnull-compare]
     opr_Assert(s != NULL && d != NULL);
                               ^
/…/openafs/include/afs/opr.h:28:15: note: in definition of macro ‘__opr_Assert’
     do {if (!(ex)) opr_AssertionFailed(__FILE__, __LINE__);} while(0)
               ^~
casestrcpy.c:26:5: note: in expansion of macro ‘opr_Assert’
     opr_Assert(s != NULL && d != NULL);
     ^~~~~~~~~~
casestrcpy.c:26:18: error: nonnull argument ‘s’ compared to NULL [-Werror=nonnull-compare]
     opr_Assert(s != NULL && d != NULL);
                  ^
/…/openafs/include/afs/opr.h:28:15: note: in definition of macro ‘__opr_Assert’
     do {if (!(ex)) opr_AssertionFailed(__FILE__, __LINE__);} while(0)
               ^~
casestrcpy.c:26:5: note: in expansion of macro ‘opr_Assert’
     opr_Assert(s != NULL && d != NULL);
     ^~~~~~~~~~
casestrcpy.c: In function ‘opr_ucstring’:
casestrcpy.c:46:31: error: nonnull argument ‘d’ compared to NULL [-Werror=nonnull-compare]
     opr_Assert(s != NULL && d != NULL);
                               ^
/…/openafs/include/afs/opr.h:28:15: note: in definition of macro ‘__opr_Assert’
     do {if (!(ex)) opr_AssertionFailed(__FILE__, __LINE__);} while(0)
               ^~
casestrcpy.c:46:5: note: in expansion of macro ‘opr_Assert’
     opr_Assert(s != NULL && d != NULL);
     ^~~~~~~~~~
casestrcpy.c:46:18: error: nonnull argument ‘s’ compared to NULL [-Werror=nonnull-compare]
     opr_Assert(s != NULL && d != NULL);
                  ^
/…/openafs/include/afs/opr.h:28:15: note: in definition of macro ‘__opr_Assert’
     do {if (!(ex)) opr_AssertionFailed(__FILE__, __LINE__);} while(0)
               ^~
casestrcpy.c:46:5: note: in expansion of macro ‘opr_Assert’
     opr_Assert(s != NULL && d != NULL);
     ^~~~~~~~~~
casestrcpy.c: In function ‘opr_strcompose’:
/…/openafs/include/afs/opr.h:28:12: error: nonnull argument ‘buf’ compared to NULL [-Werror=nonnull-compare]
     do {if (!(ex)) opr_AssertionFailed(__FILE__, __LINE__);} while(0)
            ^
/…/openafs/include/afs/opr.h:37:25: note: in expansion of macro ‘__opr_Assert’
 # define opr_Assert(ex) __opr_Assert(ex)
                         ^~~~~~~~~~~~
casestrcpy.c:98:5: note: in expansion of macro ‘opr_Assert’
     opr_Assert(buf != NULL);
     ^~~~~~~~~~
kalocalcell.c: In function ‘ka_CellToRealm’:
/…/openafs/include/afs/opr.h:28:12: error: nonnull argument ‘realm’ compared to NULL [-Werror=nonnull-compare]
     do {if (!(ex)) opr_AssertionFailed(__FILE__, __LINE__);} while(0)
            ^
/…/openafs/include/afs/opr.h:37:25: note: in expansion of macro ‘__opr_Assert’
 # define opr_Assert(ex) __opr_Assert(ex)
                         ^~~~~~~~~~~~
kalocalcell.c:117:5: note: in expansion of macro ‘opr_Assert’
     opr_Assert(realm != NULL);
     ^~~~~~~~~~

Change-Id: I6fd618ed49255d7b3de2f8f3424d9659890829c0
Reviewed-on: https://gerrit.openafs.org/12442
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>