environment: Remove old compatibility code
St.set_slow_down_factor() has been deprecated for over 3 years (since 3.34). That's plenty of time for extensions to switch to St.Settings.get().slow_down_factor.
St.set_slow_down_factor() has been deprecated for over 3 years (since 3.34). That's plenty of time for extensions to switch to St.Settings.get().slow_down_factor.