(svn r7575) -Codechange: Allow the AI to build signals under bridges (revert r4911)

BUGS

@@ -10,6 +10,3 @@ Normal and elrail depots look the same. Use 'X' (transparent buildings)
    to distinguish between them
 Missing curors / icons for construction (currently using the conventional ones)
 
-TODO:
-- Reallow the AI to build signals under bridges (about ai/default/default.c:1600) as
-  r4911 manually disallows it (not via the command)

ai/default/default.c

@@ -1615,9 +1615,8 @@ clear_town_stuff:;
 
 			/* signals too? */
 			if (j & 3) {
-				/* XXX - we need to check manually whether we can build a signal if DC_EXEC is
-				   not set because the rail has not actually been built */
-				if (!IsTileType(c, MP_RAILWAY)) return CMD_ERROR;
+				// Can't build signals on a road.
+				if (IsTileType(c, MP_STREET)) return CMD_ERROR;
 
 				if (flag & DC_EXEC) {
 					j = 4 - j;