MEDIUM: mux-h2: Stop dealing with HTX flags transfer in h2_rcv_buf()

MEDIUM: mux-h2: Stop dealing with HTX flags transfer in h2_rcv_buf()

In h2_rcv_buf(), HTX flags are transfer with data when htx_xfer() is
called. There is no reason to continue to deal with them in the H2 mux. In
addition, there is no reason to set SE_FL_EOI flag when a parsing error was
reported. This part was added before the stconn era. Nowadays, when an HTX
parsing error is reported, an error on the sedesc should also be reported.