Update tsearch regex memory management.

Update tsearch regex memory management.

Now that our regex engine uses palloc(), it's not necessary to set up a
special memory context callback to free compiled regexes.  The regex has
no resources other than the memory that is already going to be freed in
bulk.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CA%2BhUKGK3PGKwcKqzoosamn36YW-fsuTdOPPF1i_rtEO%3DnEYKSg%40mail.gmail.com