Proactively remove libc++ __tuple header file that became a directory

This was already handled in ObsoleteFiles.inc (see the 20230622 entry),
but some people never run "make delete-old", or want to upgrade directly
from a revision that still had libc++ 15 to the most recent revision.

They would then encounter a failure during installworld, similar to:

install: target directory `/usr/include/c++/v1/__tuple/' does not exist

Therefore, clean it up in the distrib-cleanup phase, similar to the
earlier instance of the libc++ __string header file that became a
directory.

PR: 273753
MFC after: 1 month