Simplify PME step workload management

Simplify PME step workload management

Used StepWorkload further down into the PME code, rather than the
custom GMX_PME_* flags.

GMX_PME_CALC_POT was unused, so removed.

Spread, solve, and backFFT were always performed, so removed GMX_PME_*
flags and related booleans for them. Re-indented code that was behind
these branches that were always taken. Note that we don't always
calculate forces in PME (currently TPI, but perhaps soon also MC).

Change-Id: Ia7019a0042bde9ac04c3beacbf05f7af41c7ed3c