pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ebaaf386adb133010c2024256521b993c5e53e98

Modified Files
--------------
src/interfaces/libpq-oauth/Makefile          |  14 -
src/interfaces/libpq-oauth/meson.build       |  35 --
src/interfaces/libpq-oauth/t/001_oauth.pl    |  24 --
src/interfaces/libpq-oauth/test-oauth-curl.c | 527 ---------------------------
4 files changed, 600 deletions(-)