Remove documentation for --enable-dotnet-grades from configure.

The presence of the option --enable-dotnet-grades in the configure options
is confusing users.  Hide the documentation for it.

Delete the configure option --enable-minimal-hlc-trail-install which was
only ever added as a shortcut for members of the G12 project.  It's just
cluttering things up now.

configure.ac:
.INSTALL.in:
	As above.
Merge remote-tracking branch 'upstream/master' into feature-char-lead-trail-surrogate