vala: deprecate target package

Vala generates C code from Vala code, and therefore requires a
compiler. Since we are deprecating the native compiler support on the
target, having vala on the target doesn't make sense.

Of course, we keep vala for the host, where compilation happens.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>