QNX: Improve patch_qnx6_prl_files() function

Use bldinstallercommon.locate_directory() to find the 'lib' directory
instead of kind of guessing it using INSTALL_PREFIX.

Also remove some useless validations about checking whether 'lib' exists
and whether the .prl files exist.

Change-Id: I5f19098601b61380fc14e74e062660d2d870c7c5
Reviewed-by: Wolfgang Bremer <wbremer@blackberry.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>