tile: Remove modified-time and etag

Previously, it was useful to have these properties on ShumateTile so
they could be passed between sources in a ShumateMapSourceChain. Now
that's gone, the file cache and network source communicate directly, and
these properties are no longer needed.
1 job for map-source-cleanup in 2 minutes and 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #1242984
fedora

00:02:57

51.0%