GDScript: Consolidate behavior for assigning enum types

This makes sure that assigning values to enum-typed variables are
consistent. Same enum is always valid, different enum is always
invalid (without casting) and assigning `int` creates a warning
if there is no casting.

There are new test cases to ensure this behavior doesn't break in
the future.

modules/gdscript/gdscript_compiler.cpp

@@ -141,7 +141,6 @@ GDScriptDataType GDScriptCompiler::_gdtype_from_datatype(const GDScriptParser::D
 			}
 		} break;
 		case GDScriptParser::DataType::ENUM:
-		case GDScriptParser::DataType::ENUM_VALUE:
 			result.has_type = true;
 			result.kind = GDScriptDataType::BUILTIN;
 			result.builtin_type = Variant::INT;