parser: revert silent _ prefix rule

vlib/encoding/csv/reader_test.v

@@ -112,7 +112,7 @@ fn test_no_line_ending() {
 
 	mut row_count := 0
 	for {
-		_row := csv_reader.read() or {
+		csv_reader.read() or {
 			break
 		}
 		row_count++

vlib/v/parser/parser.v

@@ -197,9 +197,6 @@ pub fn (mut p Parser) open_scope() {
 pub fn (mut p Parser) close_scope() {
 	if !p.pref.is_repl && !p.scanner.is_fmt {
 		for v in p.scope.unused_vars() {
-			if v.name.len > 0 && v.name[0] == `_` {
-				continue
-			}
 			if p.pref.is_prod {
 				p.error_with_pos('Unused variable: $v.name', v.pos)
 			} else {