cmake: drop OE specific environment variable support

CMAKE_TOOLCHAIN_FILE is now also a supported environment
variable, we can export it directly. There's no need to
use a OE specific patch to add OE_CMAKE_TOOLCHAIN_FILE
support.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>