caps: Remove the FreeBSD implementation of pa_drop_caps()

caps: Remove the FreeBSD implementation of pa_drop_caps()

Quoting Ryan Lortie from [1]:

I assumed from my reading of the Linux code ("cap_clear()...") that it
was clearing all capabilities of the process when in fact it is only
clearing the "special to root" capabilities.

The FreeBSD version of the code indeed clears _all_ capabilities beyond
ones that the process already has (ie: cannot open any new files, create
sockets, etc.)

This has a pretty obvious adverse effect on pulseaudio's ability to do
what it needs to do -- indeed, it bombs out pretty quickly due to an
inability to read its own config file.

[1] https://bugs.freedesktop.org/show_bug.cgi?id=72580#c11