Merge pull request #170 from billsacks/glc_grid

Remove valid_values for GLC_GRID
### Description of changes

Remove valid_values for GLC_GRID.

This way we don't need to change this xml entry whenever we want to add
a new GLC grid. (None of the other *_GRID variables define valid_values.)

Also, set default_value to UNSET, as for other *_GRID variables.

### Specific notes

Contributors other than yourself, if any: none

CMEPS Issues Fixed (include github issue #): none

Are changes expected to change answers?
 - [x] bit for bit
 - [ ] different at roundoff level
 - [ ] more substantial 

Any User Interface Changes (namelist or namelist defaults changes)?
 - [ ] Yes
 - [x] No

Testing performed if application target is CESM:(either UFS-S2S or CESM testing is required):
- [ ] (recommended) CIME_DRIVER=nuopc scripts_regression_tests.py
   - machines:
   - details (e.g. failed tests):
- [ ] (recommended) CESM testlist_drv.xml
   - machines and compilers:
   - details (e.g. failed tests):
- [ ] (optional) CESM prealpha test
   - machines and compilers
   - details (e.g. failed tests):
- [x] (other) please described in detail
  - `ERS_Vnuopc_Ly7.f09_g17_gl4.T1850G.cheyenne_intel`: passes
  - `ERS_Vnuopc_Ly3.f10_f10_mg37.I1850Clm50SpG.cheyenne_intel`: fails with runtime error that seems unrelated to these changes; the important point, I think, is that setup passes and GLC_GRID is set correctly in this test (however, this issue will need to be resolved soon)

Testing performed if application target is UFS-coupled:
- [ ] (recommended) UFS-coupled testing
   - description:
   - details (e.g. failed tests):

Testing performed if application target is UFS-HAFS:
- [ ] (recommended) UFS-HAFS testing
   - description:
   - details (e.g. failed tests):

Hashes used for testing:
- [x] CESM:
  - repository to check out: https://github.com/ESCOMP/CESM.git
  - branch: nuopc_dev
  - hash: `913272a`
- [ ] UFS-coupled, then umbrella repostiory to check out and associated hash:
  - repository to check out:
  - branch:
  - hash:
- [ ] UFS-HAFS, then umbrella repostiory to check out and associated hash:
  - repository to check out:
  - branch:
  - hash:

cime_config/config_component.xml

@@ -1177,8 +1177,7 @@
 
   <entry id="GLC_GRID">
     <type>char</type>
-    <valid_values>gland20,gland10,gland5,gland5UM,gland4,mpas.gis20km,mpas.ais20km,null</valid_values>
-    <default_value>gland5UM</default_value>
+    <default_value>UNSET</default_value>
     <group>build_grid</group>
     <file>env_build.xml</file>
     <desc>glacier (glc) grid - DO NOT EDIT (for experts only)</desc>