remove Externals.cfg, add submodules

[jedwards4b]

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Description of changes

Removes requirement for manage_externals, uses git submodules instead.

Specific notes

Contributors other than yourself, if any:

CDEPS Issues Fixed (include github issue #):

Are there dependencies on other component PRs (if so list):

Are changes expected to change answers (bfb, different to roundoff, more substantial):

Any User Interface Changes (namelist or namelist defaults changes):

Testing performed (e.g. aux_cdeps, CESM prealpha, etc):

Hashes used for testing: