SUNRPC: Remove unused krb5_decrypt

The last use of krb5_decrypt() was removed in 2023 by
commit 2a9893f7 ("SUNRPC:
Remove net/sunrpc/auth_gss/gss_krb5_seqnum.c")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>