mesa-gl: make recipe target only

mesa-gl: make recipe target only

According to the introducing commit log[1] and Dmitry's recollection[2],
the whole point of mesa-gl recipe is to provide GL library in case there
are vendor-provided GLES libraries.

Therefore, let's make this recipe target only by removing the
BBCLASSEXTEND variable.

No intended change in behavior for the target recipe.

[1] https://git.yoctoproject.org/poky/commit/?id=015cb13a67c672de30f5384dab5ab4b8db305281
[2] https://lore.kernel.org/openembedded-core/5ebxxyvkcur3zpef5krvyizomgdgtls4qau7s2i2mgcmvs2loy@ilcud37qk6sn/

Suggested-by: Ross Burton <ross.burton@arm.com>
Suggested-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Tested-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>