LibWeb: Correct escape handling in CSS Tokenizer

Calling is_valid_escape_sequence() with no arguments hides what it
is operating on, so I have removed that, so that you must explicitly
tell it what you are testing.

The call from consume_a_token() was using the wrong tokens, so it
returned false incorrectly. This was resulting in corrupted output
when faced with this code from Acid2. (Abbreviated)

```css
.parser { error: \}; }
.parser { }
```

Userland/Libraries/LibWeb/CSS/Parser/Tokenizer.h

@@ -92,7 +92,6 @@ private:
     void consume_the_remnants_of_a_bad_url();
     void consume_comments();
     void reconsume_current_input_code_point();
-    [[nodiscard]] bool is_valid_escape_sequence();
     [[nodiscard]] static bool is_valid_escape_sequence(U32Twin);
     [[nodiscard]] bool would_start_an_identifier();
     [[nodiscard]] bool would_start_an_identifier(U32Triplet);