If we avoid calling pg_regress with an empty --multibyte we
can skip GPDB specific handling of the encoding parameter in
pg_regress and instead follow upstream. While the local diffs
to pg_regress weren't wrong per se, avoiding merge conflicts
is of more interest.

Also revert the commented out MULTIBYTE in the makefile to
match the upstream value. A follow-up commit will fix this
as well.