main: Removes _mesa_unpack_bitmap() in favor of the more generic func _mesa_unpack_image()

_mesa_unpack_bitmap() was introduced by commit 02b801c1edec6400a4192e3e5b0595b13b771b18
to handle the case when data is stored in PBO by display lists, in the context of this bug:

Incorrect pixels read back if draw bitmap texture through Display list
https://bugs.freedesktop.org/show_bug.cgi?id=10370

Since _mesa_unpack_image() already handles the case of GL_BITMAP, this patch removes
_mesa_unpack_bitmap() and makes affected calls to go through _mesa_unapck_image()
instead.

The sample test attached to the original bug report passes with this change, and there are no
piglit regressions.

Signed-off-by: Eduardo Lima Mitev <elima@igalia.com>