Revert "Build PostgreSQL with XML support on FreeBSD"

Summary:
FreeBSD package is now built by default with XML support,
we don't need to build it manually anymore.

This reverts commit 7406ef91eb10d1df04e56c5b185a6ec05abe33e4.
This reverts commit e79d77e83a8562e61b725bfabad6037e0610c993.

Ref T2083

Test Plan:
ldd /usr/local/bin/postgres shows a link to libxml2:
libxml2.so.16 => /usr/local/lib/libxml2.so.16 (0x4847274e000)

Maniphest Tasks: T2083

Differential Revision: https://devcentral.nasqueron.org/D3882