Revert external frame buffer code.

Revert external frame buffer code.

A future CL will add external frame buffers
differently.

Squash commit of four revert commits:
Revert "Increase required number of external frame buffers"

This reverts commit 9e41d569d7c84dd9ca8f0047c15377a883945685.

Revert "Add external constants."

This reverts commit bbf53047b03106e3c2e24b28cb836cc838db5ee8.

Revert "Add frame buffer lru cache."

This reverts commit fbada948fa345e67acf9aa41a8f9a78f5dfe8648.

Conflicts:
	vpxdec.c

Change-Id: I76fe42419923a6ea6c75d9997cbbf941d73d3005

Revert "Add support to pass in external frame buffers."

This reverts commit 10f891696bc4c972c13cc9fde2c53470501a03e2.

Conflicts:
	test/external_frame_buffer_test.cc
	vp9/common/vp9_alloccommon.c
	vp9/common/vp9_reconinter.c
	vp9/decoder/vp9_decodeframe.c
	vp9/encoder/vp9_onyx_if.c
	vp9/vp9_dx_iface.c
	vpx/vpx_decoder.h
	vpx/vpx_external_frame_buffer.h
	vpx_scale/generic/yv12config.c
	vpxdec.c

Change-Id: I7434cf590f1c852b38569980e4247fad0d939c2e