Revert "oauth: Add unit tests for multiplexer handling"

Revert "oauth: Add unit tests for multiplexer handling"

Commit 1443b6c0e introduced buildfarm breakage for Autoconf animals,
which expect to be able to run `make installcheck` on the libpq-oauth
directory even if libcurl support is disabled. Some other Meson animals
complained of a missing -lm link as well.

Since this is the day before a freeze, revert for now and come back
later.

Discussion: https://postgr.es/m/CAOYmi%2BnCkoh3zB%2BGkZad44%3DFNskwUg6F1kmuxqQZzng7Zgj5tw%40mail.gmail.com