clean up MERRA-2 restart option

[weiyuan-jiang]

This PR simplified the check on RESTART='M'.

Related PRs:
GEOS-ESM/GEOS_Util#188

[biljanaorescanin]

Testing summary:

PR was tested in combination with GEOS_Util 188
All regression tests passed.

For functionality:
Testing M2 RESTART option from Discover36 worked.
If we try same setup from Discover32 user will see:

File "/gpfsm/dnb34/borescan/brisi/pr188_util/GEOSldas/install/bin/remap_utils.py", line 155, in init_merra2
assert os.path.exists(MERRA2_RST_BASE), "Must be on discover30 or discover36 to access MERRA-2 ``restarts at " + MERRA2_RST_BASE
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: Must be on discover30 or discover36 to access MERRA-2 restarts at ``/archive/users/gmao_ops/MERRA2/gmao_ops/GEOSadas-5_12_4/