Or alternatively: Revert "Revert "Revert "Remove vala from the runtime"""

This reverts commit ad6fb4b7.

Chris is reporting that vapigen runs without actually generating vapi
files:

/usr/bin/install -c -m 644 ./gmime-3.0.vapi gmime-3.0.deps '/app/share/vala/vapi'
/usr/bin/install: cannot stat './gmime-3.0.vapi': No such file or directory

We'll need some more time to figure out what's going wrong.

Part-of: <!2231>