chromium: drop browser and mksnapshot packaging.

To reduce how much it is built with Chromium, just remove a couple of
parts that are not used:
* Chromium browser is not used or packaged, and we intend to use a lighter
version with app-shell and enact browser likely.
* mksnapshot cross package was intended for optimizing launch time of
certain web applications. But we are not using it yet.

Bug-AGL: SPEC-4391
Signed-off-by: Jose Dapena Paz <jdapena@igalia.com>
Change-Id: Id9f15c42c81341962f1f0c7a91b31824f116a9ef