bitbake.conf: drop GPE_MIRROR variable

bitbake.conf: drop GPE_MIRROR variable

GPE_MIRROR is not used by any recipes. Beside this it looks like
the url (http://gpe.linuxtogo.org/download/source) is unavailable,
and the main url (linuxtogo.org) is using an invalid certificate
(issued for another website).

It looks this mirror isn't useful nor valid anymore - so drop it.

Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>