Revert all new recent changes to add PGXS options for TAP and isolation

Revert all new recent changes to add PGXS options for TAP and isolation

A set of failures in buildfarm machines are proving that this is not
quite ready yet because of another set of issues:
- MSVC scripts assume that REGRESS_OPTS can only use top_builddir.  Some
test suites actually finish by using top_srcdir, like pg_stat_statements
which cause the regression tests to never run.
- Trying to enforce top_builddir does not work either when using VPATH
as this is not recognized properly.
- TAP tests of bloom are unstable on various platforms, causing various
failures.