targets/nine: remove the custom pipe-driver path management

targets/nine: remove the custom pipe-driver path management

Since the up-streaming of nine, the static target was used by default.
The dynamic pipe-drivers being available only via manual tweak of
configure.ac.

As we'll be removing the library_path argument from the pipe-loader with
follow-up commits, we can remove D3D9_DRIVERS_PATH/D3D9_DRIVERS_DIR.
Everyone doing local hacking on nine, or wishing to have a env override
can bring them back within the pipe-loader.

Cc: Axel Davy <axel.davy@ens.fr>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Rob Clark <robclark@freedesktop.org>