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
```

renderer/HardwareTransforms.cpp

@@ -116,4 +116,4 @@ void g3_RefreshTransforms(bool usePassthru) {
   sUseTransformPassthru = (usePassthru) ? 1 : 0;
 }
 
-#endif
+#endif