docs/: disabed additional index entries again, as this makes docs-gen just slow and they aren't useful yet

Original commit message from CVS:
* docs/gst/gstreamer-docs.sgml:
* docs/libs/gstreamer-libs-docs.sgml:
disabed additional index entries again, as this makes docs-gen just
slow and they aren't useful yet
* docs/libs/gstreamer-libs-sections.txt:
little -section.txt cleanup for libs