Remove blocktype thumb.png files (Bug 1532642)

Since we're no longer using theme in any of the core themes.

Note that the code to look for a thumb.png file is still in place,
for backwards-compatibility with older 3rd-party blocktypes.

If you're a third-party theme developer who wants to use PNG files
for the block picker instead of FontAwesome, you'll need to provide
your own thumb.png files for each of the core blocktypes, under
your theme's /theme/THEMENAME/plugintype... directories.

behatnotneeded: Covered by existing tests

Change-Id: I6f0578c8f55c620b7b3df42d8959a6d1ea5ad016