Removed more stuff related to gmx.ff and gmx2.ff

Code and documentation no longer refers to these force fields. There
are corresponding changes to examples in the manual.

Updated list of force fields in the manual, plus minor correction
to description of CHARMM27 there and in the forcefield.doc.

It would be nice to leave gmx2?.ff/forcefield.itp intact with an error
message, but this means gmx pdb2gmx continues to show these force
fields. Instead, I made a README to note that these force fields have
gone.

Change-Id: I10480a0fad169493d13a743c43e8b2321128155d