i965: Drop broken front_buffer_reading/drawing optimization.

i965: Drop broken front_buffer_reading/drawing optimization.

The flag wasn't getting updated correctly when the ctx->DrawBuffer or
ctx->ReadBuffer changed.  It usually ended up working out because most
apps only have one window system framebuffer, or if they have more than
one and they have any front read/drawing, they will have called
glReadBuffer()/glDrawBuffer() on it when they get started on the new
buffer.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>