Remove unused method NativePixmap::GetEGLClientBuffer()

GbmBuffer used to have it implemented [1] but it's empty now [2]
and is returning nullptr elsewhere.

[1] https://codereview.chromium.org/365193003/diff/60001/ui/ozone/platform/dri/gbm_buffer.cc
[2] https://cs.chromium.org/chromium/src/ui/ozone/platform/drm/gpu/gbm_buffer.cc?q=gbmbuffer&sq=package:chromium&dr=CSs&l=331

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I98d017c5dbbef27cf24454cd44ee519749639507
Reviewed-on: https://chromium-review.googlesource.com/1071947
Reviewed-by: Julien Isorce <julien.isorce@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561878}

• ui/gfx/linux/native_pixmap_dmabuf.cc[diff]
• ui/gfx/linux/native_pixmap_dmabuf.h[diff]
• ui/gfx/native_pixmap.h[diff]
• ui/gl/gl_image_native_pixmap.cc[diff]
• ui/ozone/platform/cast/surface_factory_cast.cc[diff]
• ui/ozone/platform/drm/gpu/gbm_buffer.cc[diff]
• ui/ozone/platform/drm/gpu/gbm_buffer.h[diff]
• ui/ozone/platform/headless/headless_surface_factory.cc[diff]