Change MOM_generic_tracer to a stub (mom-ocean#884)

* Change MOM_generic_tracer into a stub

This PR removes the content from the functions and routines in MOM_generic_tracer to turn it into a stub.
In addition, the other modules in config_src/external/GFDL_ocean_BGC are removed since they are no longer called anywhere.

* Clean up module use statements

Remove unneeded use statements.

* Add fatal error if stub code is called

A fatal error has been added to register_MOM_generic_tracer which
will occur if the stub version of MOM_generic_tracer has been
compiled but use_generic_tracer is true.

* Fix missing '&' at continued line

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Co-authored-by: Theresa Cordero <Theresa.Cordero@gaea65.ncrc.gov>
Co-authored-by: Theresa Cordero <Theresa.Cordero@gaea63.ncrc.gov>

Author: 3 people