GlobalVisTestNonRemovableHorizon() was only used for the implementation of
snapshot_too_old, which was removed in f691f5b80a. As using
GlobalVisTestNonRemovableHorizon() is not particularly efficient, no new uses
for it should be added. Therefore remove.
Discussion: https://postgr.es/m/20240415185720.q4dg4dlcyvvrabz4@awork3.anarazel.de
3ab8cf9275