Remove useless pg_audit.conf

No need to have pg_audit.conf any longer since the regression tests are
just loading the module at the start of each session (to simulate being
in shared_preload_libraries, which isn't something we can actually make
happen on the buildfarm itself, it seems).

Pointed out by Tom

contrib/pg_audit/pg_audit.conf

-shared_preload_libraries = pg_audit