This was GDB-only. It complicates the startup mechanism including breaking
certain setups (e.g. users manually controlling "set auto-solib-add" in
their .gdbinit) without providing too much benefit (anymore).

Part of the potential debugger startup time improvements that were
possibly gained by the feature are nowadays available by using a
release build *without* debug info, as one of the original reasons
to not use that (making Qt type introspection hard/impossible)
is largely gone with 4.2+.

Change-Id: Iffb34e0035f39ad74b24bf025749b546f25a489b
Reviewed-by: Christian Stenger <christian.stenger@qt.io>