Clean up unused code from gmxfio

Clean up unused code from gmxfio

- gmx_fio_do_*() are only used for writing xdr-based formats, so remove
  other implementations (has been unused since support for .tpa and .tpb
  formats was removed).  Further decoupling should be possible, but
  left for later to keep this change smaller.
- gmx_fio_read/write_*() were not used with the exception of
  gmx_fio_write_string(), so removed.  gmx_fio_do_*() should be
  sufficient.
- Remove USE_XDR, as it is not feasible to turn off xdr support from
  these few files alone.  There are several file formats (most notably,
  run input files and checkpoints) that can only be written in xdr, and
  functionality is severely limited without those.
- Remove unused (and mistyped) HAVE_XML #ifdefs.  It's not feasible to
  inject XML support at this level...
- Move confusingly placed check for not opening .tng files with gmxfio.
- Removed unnecessary checks for debug files (debug files are no longer
  opened with gmx_fio_fopen()).
- Remove a few other fields, enum members, and methods that had no effect.
- Rename gmxfio_int.h to gmxfio-impl.h to make its role clearer and to
  make it follow the current naming conventions.
- Remove unnecessary #includes.

Change-Id: Ida39a076969da116c9a5f68d7bfd0355c2d2363b