Move physics.* to math/units.*

- Remove some unnecessary #includes.
- Remove the dummy tests; easy to rewrite as usable tests using GTest if
  someone needs those.  They were anyways testing code that no one is
  still calling, even though it was introduced several years ago.
  Now gmxlib is free of *_test.c files.

Now, only one file using atom_id is keeping math/ dependent on
legacyheaders/.

Part of #1415.

Change-Id: I31312b92cf9f5d5129fbd90f31fcf27be2f7afaf