GDScript: Make using return of void function an error

Remove the `VOID_ASSIGNMENT` warning since those cases will be errors
now.

modules/gdscript/tests/scripts/parser/warnings/void_assignment.gd

@@ -1,6 +0,0 @@
-func i_return_void() -> void:
-	return
-
-
-func test():
-	var __ = i_return_void()

modules/gdscript/tests/scripts/parser/warnings/void_assignment.out

@@ -1,5 +0,0 @@
-GDTEST_OK
->> WARNING
->> Line: 6
->> VOID_ASSIGNMENT
->> Assignment operation, but the function 'i_return_void()' returns void.