citext: Allow tests to pass in OpenSSL FIPS mode

citext: Allow tests to pass in OpenSSL FIPS mode

citext doesn't define an md5() function, so the value of using it in
its tests is dubious.  At best this shows in an indirect way that the
cast from citext to text works.  Avoid the issue and remove the test.

Discussion: https://www.postgresql.org/message-id/flat/dbbd927f-ef1f-c9a1-4ec6-c759778ac852%40enterprisedb.com