Remove warnings and docs for ancient preview problems.

* doc/preview-problems.texi: Rename "Known problems" section to
"Known problems with preview-latex" in rawfile export.  Delete
sections "Problems with Ghostscript" and "Emacs problems" because
those are basically ruled out when using the prerequisites
documented in the manual.

* doc/install.texi: Don't link to ghostscript specific
preview-latex problems section because that has been removed.

* doc/Makefile.in (DISTTEXTS, ../PROBLEMS.preview): Rename
PROBLEMS to PROBLEMS.preview since it only deals with
preview-latex problems.

* Makefile.in (DISTTEXTS): Use PROBLEMS.preview instead of
PROBLEMS.

* preview/Makefile.in (use-hint): Don't warn about problems with overly
obsolete packages.