MINOR: mux-h1: clean up conditions to enabled and disabled splicing

MINOR: mux-h1: clean up conditions to enabled and disabled splicing

First, there is no reason to announce the splicing support at the
conn-stream level when it is created, at least for now. GTUNE_USE_SPLICE
option is already handled at the stream level.

Second, in h1_rcv_buf(), there is no reason to test the message state to
switch the H1C in splicing mode (via H1C_F_WANT_SPLICE flag).
h1_process_input() already takes care to set CS_FL_MAY_SPLICE flag on the
conn-stream when appropriate. Thus, in h1_rcv_buf(), we can rely on this
flag to change the H1C state.

Finally, if h1_rcv_pipe() is called, it means the H1C is already in the
splicing mode. H1C_F_WANT_SPLICE flag is necessarily already set. Thus no
reason to force it.