Clean up public interfaces in field_manager_mod and other modules

[mlee03]

Is your feature request related to a problem? Please describe.
The module procedures within an interface are declared as public along with the interface in field_manager and fm_util . For example,

public test
public test_integer
public test_real

interface  test
  module procedure test_integer
  module procedure test_real
end interface test 

The above should be

public test
interface  test
  module procedure test_integer
  module procedure test_real
end interface test 

Describe the solution you'd like
Cleanup of the interfaces in FMS

Describe alternatives you've considered

Additional context
In order to make FMS more perfect

[thomas-robinson]

Are test_real and test_integer routines that was recently added? How do we know that no one is calling them?

[mlee03]

It seems like none of the model components call test_real and test_integer, i.e, ocean_BGC calls fm_get_value not fm_get_value_real, etc.