egl: Remove the 565 pbuffer-only EGL config under X11.

The CTS finally has agreed to drop the requirement for a
565-no-depth-no-stencil config for ES 3.0. Hence we can now remove the
code to satisfy this requirement using a pbuffer-only visual with
whatever other buffers the driver happens to have given us.

This reverts commit 82607f8a,
commit 6ad31c4f and
commit dacb11a5.

v2:
  - Reference the VK-GL-CTS issue (Eric E.).

v3:
  - Don't revert
    fc21394b ("egl: Quiet warning about front buffer rendering for pixmaps/pbuffers")
    (Kenneth).

References: VK-GL-CTS issue 1601.
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Andres Gomez <agomez@igalia.com>
Acked-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit 02c265be)
Conflicts resolved by Dylan Baker