MINOR: lua: Remove the flag HLUA_TXN_HTTP_RDY

MINOR: lua: Remove the flag HLUA_TXN_HTTP_RDY

This flag was used in some internal functions to be sure the current stream is
able to handle HTTP content. It was introduced when the legacy HTTP code was
still there. Now, It is possible to rely on stream's flags to be sure we have an
HTX stream.

So the flag HLUA_TXN_HTTP_RDY can be removed. Everywhere it was tested, it is
replaced by a call to the IS_HTX_STRM() macro.

This patch is mandatory to allow the support of the filters written in lua.