py/lexer: Remove unnecessary check for EOF in lexer's next_char func.

This check always fails (ie chr0 is never EOF) because the callers of this
function never call it past the end of the input stream.  And even if they
did it would be harmless because 1) reader.readbyte must continue to
return an EOF char if the stream is exhausted; 2) next_char would just
count the subsequent EOF's as characters worth 1 column.

py/lexer.c

@@ -128,10 +128,6 @@ STATIC bool is_tail_of_identifier(mp_lexer_t *lex) {
 }
 
 STATIC void next_char(mp_lexer_t *lex) {
-    if (lex->chr0 == MP_LEXER_EOF) {
-        return;
-    }
-
     if (lex->chr0 == '\n') {
         // a new line
         ++lex->line;