This code is unsafe, as proven by buildfarm failures, because it tries
to access shared memory that might already be gone.  It's also unnecessary,
because we're about to exit the process anyway and so the record type cache
should never be accessed again.  The idea was to lay some foundations for
someday recycling workers --- which would require attaching to a different
shared tupdesc registry --- but that will require considerably more
thought.  In the meantime let's save some bytes by just removing the
nonfunctional code.

Problem identification, and proposal to fix by removing functionality
from the detach function, by Thomas Munro.  I went a bit further by
removing the function altogether.

Discussion: https://postgr.es/m/E1dsguX-00056N-9x@gemulon.postgresql.org