remove manage_externals

[jedwards4b]

Remove manage externals from the mizuRoute repository

Fixes #450

[ekluzek]

@jedwards4b thanks so much for putting this PR together. We appreciate your doing it.

Note, that we weren't planning on making this a requirement for cesm3_0_beta01, as the only need for externals management for mizuRoute is for the standalone version. So we figure this can come in after cesm3_0_beta01 is done.

[nmizukami]

Thanks Jim (@jedwards4b) for starting this, One thing I would like to check before merging this is:
do we have to have alternative to pull externals? Here PIO. I cannot remember what the new method is to replace manage external. git submodule??

[jedwards4b]

@nmizukami git-fleximod is the replacement. I can add it to this PR but won't do it now as apparently this update is to be deferred.