patches: Update PSTON2 to be based on new CYL1_1.

This is (I believe) the last of the TNT placeholder textures that
were using the Doom font. There were others in /patches/catoptromancy
that were in the same format (metal wall based on CYL1_1) but they
have been replaced by @AXDOOMER's latest update.

Also remove the placeholder.gif file which now seems redundant.
This fixes #102.