Remove error message hints mentioning configure options

We have some error messages like this:

SELECT xml_is_well_formed('<abc/>');
ERROR: unsupported XML feature
DETAIL: This functionality requires the server to be built with libxml
support.
HINT: You need to rebuild PostgreSQL using --with-libxml.

This patch removes these kinds of hints.

I think these hints are usually not useful since users will use packaged
distributions and won't be interested in rebuilding their installation
from source. Also, we have only used these kinds of hints for some
features and in some places, not consistently throughout. And of course
there are build systems that don't use configure. The information
"needs to be built with XXX" or "was not built with XXX" should be
enough for those interested in actually changing their build
configuration to figure out what to do.