Remove automated generation of dwarf index

The index is only helpful if the version of GDB to
create it uses the same version as the GDB version
that consumes it. Outside the "local development"
scenario this happens only by conincidence, still
we add ~3.6% to the debug library size and face
maintenance issues like QTBUG-34950.

We also don't see the same performance benefit anymore
with recent versions as we did when the feature was
added, so it's best to not create the index anymore.

People who need it, still can add it manually, or
by the 'gdb-add-index' tool that comes with recent
versions of GDB, or trust their distributors to
set up indexes matching their runtime environment.

Task-number: QTBUG-34950
Change-Id: Id4c79fa51fea9622b0891bd9b9b395b948ecb157
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>