Remove support for MapFormat < 10.

[pchote]

This would allow several obsolete parts of the code (e.g. minimap creation) to be removed. This would also be a good time to do a clean out of old maps targeting OpenRA versions more than two years outdated.

[dsimmons87]

Ok, I've created a PR to prevent uploads of maps older than map format 10.

Do you think it would be worth automatically upgrading maps where possible before cleaning out maps that it can't?

[pchote]

In the past we would do mass automatic updates on new releases, so I suspect/hope that the majority of format < 10 maps already have a format 11 revision.

#332 added the ability to filter maps by format in the admin panel - we have 2486 format 7 maps, and had two hopelessly broken format 6 maps (which I deleted). Formats 8/9/10 only existed on bleed, and there are no maps that use them. We can do a followup to #332 that adds a "newer revision has been uploaded" filter, which would give us a better idea of how many maps we can simply delete vs how many need closer inspection.

[pchote]

Technically fixed by #317, but holding off closing until we can do a mass update and then delete any revisions targeting versions older than release-20171014.