TCHAIN: remove caused_discard* argument to VG_(translate)

TCHAIN: remove caused_discard* argument to VG_(translate)
This is the followup to rev 12488.
With this revision, translation chaining is not done
if the translation with 'from address' is not existing
anymore (discarded or erased).
The assumption documented in 12488 comment has been checked by:
  * first reproduce a crash in Firefox when always setting
    caused discard to False
  * then upgrade to rev 12488
  * with this upgrade, no crash anymore.
=> this verifies that the caused discard logic is properly
replaced by revision 12488.

So, the caused discard logic can be removed.

git-svn-id: svn://svn.valgrind.org/valgrind/branches/TCHAIN@12492