don't force our runpath upon user projects anymore

now that we rely on consistently sane runpath semantics everywhere
(--enable-new-dtags on linux; the default elsewhere), there is no use
in forcing our runpath downstream: our libraries will find their
dependencies due to their embedded runpath.

this does not affect qt.prf adding qt's own library path to the user
projects' runpath.

this effectively reverts 42a7eb8d

, and some more.

Change-Id: If7af7be7b7a894bebb9b146ccb0035452223c7ac
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>