#570: Remove 'uses_forcing_file' and 'forcing_file'(_path)

Fix #570 by removing support for unimplemented legacy parameters used to configure forcings inputs.

Removals

Bmi_Adapter: delete uses_forcing_file and forcing_file (path) members, construction arguments, and accessors
BMI Formulation classes: stop passing forcing_file arguments through to BMI Adapter classes

Delete all forcing_file keys in JSON built in test fixtures
Throw errors on affirmative use of the never-active and now-removed configuration options

There may be documentation that should evolve in tandem
Users with legacy realization configs may start to see errors related to this. They've already almost certainly added the right options, since otherwise their runs just wouldn't work. So, fixing things is just a matter of removing the old bits from their files.

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