vala: remove deprecated target package

The vala target package has been deprecated since 2013.05 and thus can be
removed in 2014.05. The host vala support is left untouched.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>