py/lexer: Support raw f-strings.

Support for raw str/bytes already exists, and extending that to raw
f-strings is easy.  It also reduces code size because it eliminates an
error message.

Signed-off-by: Damien George <damien@micropython.org>

py/lexer.h

@@ -46,7 +46,6 @@ typedef enum _mp_token_kind_t {
     MP_TOKEN_LONELY_STRING_OPEN,
     #if MICROPY_PY_FSTRINGS
     MP_TOKEN_MALFORMED_FSTRING,
-    MP_TOKEN_FSTRING_RAW,
     #endif
 
     MP_TOKEN_NEWLINE,