Replace SDLTILES with just TILES

[BevapDin]

Because it's redundant and used inconsistently.

For example in sdltiles.cpp the global needupdate is only defined if SDLTILES is defined, but it's often used in places where this is not checked, e.g. CheckMessages. Similar: init_interface is not defined (but used in main.cpp) if only TILES is defined but not SDLTILES. And of course, the various make/project files already define both anyway.

I choose to keep TILES and not SDLTILES because that's what is already used in the Makefile and when invoking make TILES=1, so it's kind of consistent.