Remove MacOS (Classic) code and all references (1/3)

The MACINTOSH define refers to MacOS Classic (not OS X) which we do not
plan to support. Rather than carry the cruft forever, let's delete it.

NOTE: legacy/ is unused but we're keeping it around, so MACINTOSH uses
there are left alone.

Process used for this commit:
```
git rm -r mac
find . \( -iname "*.c" -or -iname "*.cpp" -or -iname "*.h" \) \
  -exec unifdef -UMACINTOSH -o {} {} \;
git restore legacy
git add .
```

Test Plan:
On Mac, build both Debug and Release
```
cmake --build --preset mac --config Debug
cmake --build --preset mac --config Release
```

lib/rend_opengl.h

@@ -156,10 +156,5 @@ void *opengl_DirectDrawObj();
 // returns rendering statistics for the frame
 void opengl_GetStatistics(tRendererStats *stats);
 
-#ifdef MACINTOSH
-void opengl_DettachContext(void);
-void opengl_AttachContext(CGrafPtr);
-void opengl_ResetContext(void);
-#endif
 
 #endif