Remove unused signal module

This might not make things work on macOS (we don't seem to be using the functionality in ICON anyway) but at least this should leave no undefined symbols in the library:

Undefined symbols for architecture arm64:
  "_fedisableexcept", referenced from:
      _signal_trap in util_signal.c.o
  "_feenableexcept", referenced from:
      _signal_trap in util_signal.c.o
ld: symbol(s) not found for architecture arm64

And I don't think that it would be different for an Intel-based Mac.