afs: Remove afs_osi_UnmaskUserLoop()

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

Remove the function afs_osi_UnmaskUserLoop().

The commit: 'darwin-afsdb-handler-signal-mask-20040728' (9728182c00)
introduced the function, but afs_osi_UnmaskUserLoop() 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.

Reviewed-on: https://gerrit.openafs.org/15617
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 587cfce29cb4e014e5b8d96e0433c5e7d2c6729a)

Change-Id: I0fe91a41e8f00773cb1d5976a8c3e39e4415f6bc
Reviewed-on: https://gerrit.openafs.org/15686
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_osi.c

@@ -152,14 +152,6 @@ afs_osi_MaskUserLoop(void)
 #endif
 }
 
-void
-afs_osi_UnmaskUserLoop(void)
-{
-#ifdef AFS_DARWIN_ENV
-    afs_osi_fullSigRestore();
-#endif
-}
-
 /* register rxk listener proc info */
 void
 afs_osi_RxkRegister(void)