Remove hardcoded check for "data" in level dir listing

When Ethan added PhysFS to the game, he put in a hardcoded check (marked
with a FIXME) that explicitly removed all filenames that were "data"
returned by PHYSFS_enumerateFiles(). Apparently this was due to a weird
bug with the function putting in "data" strings in its output in PhysFS
2.0.3; however, the game now uses PhysFS 3.0.2, and I could not
reproduce this bug on my system. (I also tested, and this also
straight-up ignores legitimate level filenames that just happen to be
"data" (without the .vvvvvv extension).)

After talking with Ethan in Discord DMs, I asked if we could remove this
check, and he said that we could. So I'm doing it now.

desktop_version/src/FileSystemUtils.cpp

@@ -346,10 +346,6 @@ std::vector<std::string> FILESYSTEM_getLevelDirFileNames()
 
 	for (i = fileList; *i != NULL; i++)
 	{
-		if (SDL_strcmp(*i, "data") == 0)
-		{
-			continue; /* FIXME: lolwut -flibit */
-		}
 		builtLocation = "levels/";
 		builtLocation += *i;
 		list.push_back(builtLocation);