afs: Remove afs_MemExtendEntry()

The function afs_MemExtendEntry() is not referenced anywhere within
the source tree.

Remove the function afs_MemExtendEntry().

The commit: 'memcache: add extend-entry function' (f821c7e5c8)
introduced the function, but afs_MemExtendEntry() has never been
referenced or defined within a header file.

This function is flagged due to a missing prototype when building
against a Linux 6.8 kernel (which sets the -Wmissing-declarations and
-Wmissing-prototypes compiler flags as default). Linux 6.8 commit:
 'Makefile.extrawarn: turn on missing-prototypes globally' (0fcb70851f).

When building against a kernel with CONFIG_WERROR=y, the build fails.

Note, afsMemExtendedEntry() was a wrapper for the function
_afs_MemExtendedEntry(), which is still used.

Reviewed-on: https://gerrit.openafs.org/15616
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit eab2a4ae758876bb7547d289f936f9cfc4227cf5)

Change-Id: I37f241a6df33ab15249c6708d5ca820353c8b413
Reviewed-on: https://gerrit.openafs.org/15685
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>

src/afs/afs_memcache.c

@@ -302,16 +302,6 @@ afs_MemCacheTruncate(struct osi_file *fP, int size)
     return 0;
 }
 
-int
-afs_MemExtendEntry(struct memCacheEntry *mceP, afs_uint32 size)
-{
-    int code = 0;
-    ObtainWriteLock(&mceP->afs_memLock, 560);
-    code = _afs_MemExtendEntry(mceP, size);
-    ReleaseWriteLock(&mceP->afs_memLock);
-    return code;
-}
-
 void
 shutdown_memcache(void)
 {