Resolve some TODO comments

- `out_PushSection` should not set `currentSection` to NULL because
  PUSHS, PUSHC, and PUSHO consistently keep the current section,
  charmap, and options, even though the stack has been pushed.

- `Callback__FILE__` does not need to assert that `fileName` is not
  empty because `__FILE__`'s value is quoted, and can safely be empty.

- `YY_FATAL_ERROR` and `YYLMAX` are not needed since the lexer is
  not generated with flex.

include/asm/main.h

@@ -26,12 +26,4 @@ extern bool generatedMissingIncludes;
 extern bool failedOnMissingInclude;
 extern bool generatePhonyDeps;
 
-/* TODO: are these really needed? */
-#define YY_FATAL_ERROR fatalerror
-
-#ifdef YYLMAX
-#undef YYLMAX
-#endif
-#define YYLMAX 65536
-
 #endif /* RGBDS_MAIN_H */