package/mono: remove target mips architecture

Current version of mono doesn't compile on mips due to a bug, so
disabling the mips target architecture until the bug is fixed upstream.

Fixes: http://autobuild.buildroot.net/results/b402af8cb7f00dc2e69c219926b474a8e0c8d372


and many others.

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>