Clean up some uses of String::substr

Cases where the end position is either equvalent to the default or past
the end of the string.

core/input/input.cpp

@@ -219,7 +219,7 @@ void Input::get_argument_options(const StringName &p_function, int p_idx, List<S
 				continue;
 			}
 
-			String name = pi.name.substr(pi.name.find_char('/') + 1, pi.name.length());
+			String name = pi.name.substr(pi.name.find_char('/') + 1);
 			r_options->push_back(name.quote());
 		}
 	}