Inline the effect of am/pre-inst-guile

Inline the effect of am/pre-inst-guile

It's just one variable definition, and in my opinion it confuses,
rather than helps, the overall build picture to have two names
(preinstguile and meta/guile) for the same thing.

* am/Makefile.am (am_frags): Remove pre-inst-guile.

* am/pre-inst-guile: Deleted.

* doc/ref/Makefile.am: Don't include am/pre-inst-guile.
  ($(snarf_doc).am, $(snarf_doc).texi): Expand $(preinstguile).

* module/Makefile.am (ice-9/psyntax-pp.scm.gen): Don't include
  am/pre-inst-guile.
  (ice-9/psyntax-pp.scm.gen): Expand $(preinstguile).