Always show primary tag

This implements #4935, although the wording in that issue is not correct. This commit shows primary tag on production buildings always if it is set, not only when the building is selected. Airfield has a separate tag position because of overlapping of progress bars. I experimented with different locations and I think this results in least overlapping. Airfield also has a different tag position if pixel doubling is enabled. I couldn't have done this commit without the help of WolfGaming in #openra. I tested the commit and seems to work.

I removed DrawTags method from SelectionDecorations.cs because its only purpose was to draw the primary tag which is now done in PrimaryBuilding.cs.
This will crash, because https://github.com/OpenRA/OpenRA/blob/bleed/OpenRA.Mods.RA/Fake.cs#L20 still relies on it.