Remove support for .g87 and .xyz I/O formats

Aspects of support for these have been broken since before GROMACS
4.5. That support is sometimes interactive, and thus a poor fit for
the way GROMACS supports other trajectory and coordinate formats.
Other file-format conversion options exist; GROMACS does not have to
be a Swiss-army knife.

Removed undocumented features of gmx helix that required .g87 support
for no particularly good reason.

Uncrustified files in this commit

Change-Id: Iad4845fd66ad4c55f9a31965d33ded745656db53