Roll back "Ensure the modular simulator runs only when it should"

Roll back "Ensure the modular simulator runs only when it should"

The fact that GMX_USE_MODULAR_SIMULATOR does not lead to an error
when the run is not supported is a feature, not a bug. It allows
to run the entire test set using modular simulator wherever is is
implemented by using "export GMX_USE_MODULAR_SIMULATOR=ON; make check",
even if modular simulator is disabled by default. It is the only way
to make sure that modular simulator is a valid replacement for the
legacy code path.

Similarly, using "export GMX_DISABLE_MODULAR_SIMULATOR=OFF; make check"
allows to check the legacy code path, even if modular simulator is used
by default.