package/mesa3d-demos: remove duplicate osmesa option

--disable-osmesa option is unconditionally added to CONF_OPTS even if
--enable-osmesa is used latter.

Signed-off-by: Audrey Motheron <audrey.motheron@gmail.com>
Reviewed-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>