Deprecate mdrun-only build

The following discussion from !854 (merged) should be addressed:

• @mark.j.abraham started a discussion:

  Suggest we also add a post-merge build (but in isolation it can't run the regressiontests, and I have no idea if the tests build in such a configuration). We had a build for years in Jenkins, but lost it in the move. (It never ran tests, however.)

  Frankly, I have not seen any value in the feature for years. It made more sense

  • before we had the gmx wrapper binary, and
  • when we had non-optional dependencies for analysis tools that were inconvenient to build if you were just wanting to run mdrun on the cluster

  I further suggest we discuss deprecating it so we don't have to teach users that sometimes they might see this, have less confusing documentation, and have fewer things to implement and test.