Cleanup: Remove weird assert in button handling

The function this was in already checks the conditions under which it
can operate (as it should). No need to force the caller to do these
(quite specific) checks, the function can just do nothing if the button
doesn't need these operations.
Otherwise we'd have to always execute these checks before calling, which
makes calling it a hassle, makes the code harder to follow and generally
harder to maintain (what if the conditions change?).

source/blender/editors/interface/interface.c

@@ -3342,9 +3342,6 @@ void ui_but_range_set_soft(uiBut *but)
       but->softmax = max_ff(but->softmax, value);
     }
   }
-  else {
-    BLI_assert(0);
-  }
 }
 
 /* ******************* Free ********************/