[Maintenance] `#pragma once` has no effect?

Should have remembered this from all the times I've looked at the list
of #pragma directives supported by Turbo C++… Still, I would have at
least expected a warning, but not even the `ill` warning covers this.

Part of P0062, funded by Touhou Patch Center.

defconv.h

@@ -1,7 +1,5 @@
 // Common calling convention macro for some functions that originated in TH02
 // as __cdecl, but were then changed to __pascal in TH03.
-#pragma once
-
 #ifndef GAME
 # error GAME not defined
 #elif GAME >= 3

th03/math/subpixel.hpp

@@ -1,8 +1,6 @@
 // Fixed-point format for expressing world-space coordinates, with 4 bits of
 // fractional resolution.
 
-#pragma once
-
 typedef int subpixel_t;
 
 inline subpixel_t to_sp(float screen_v) {