remove ffi.gc for http_parser_url

[gary8520]

luajit may manage http_parser_url struct directly instead of managing a struct http_parser_url * and a block of struct http_parser_url instance.

[MarcoChen97]

this change can fix the following test case by the luajit 2.0.5

self.request.headers.http_parser_url = nil
print("[Debug] url field:", self.request.headers:get_url_field(turbo.httputil.UF.PATH))

collectgarbage("collect")
print("[Debug] url field:", self.request.headers:get_url_field(turbo.httputil.UF.PATH))

before adding this change, the second print cannot get the value since the data seems to be freed by lua GC.

[gary8520]

@kernelsauce is there any comment about the PR?