Revert "Update `%pure-parser` to `%define api.pure` for newer `bison`; fix #4706"

The `bison` on MacOS does not support `%define api.pure`.
This reverts commit f7c9f1f7.

mrbgems/mruby-compiler/core/parse.y

@@ -1280,7 +1280,7 @@ heredoc_end(parser_state *p)
 
 %}
 
-%define api.pure
+%pure-parser
 %parse-param {parser_state *p}
 %lex-param {parser_state *p}