MINOR: proto_htx: Don't adjust transaction mode anymore in HTX analyzers

MINOR: proto_htx: Don't adjust transaction mode anymore in HTX analyzers

Because the option http-tunnel is now ignored in HTX, there is no longer any
need to adjust the transaction mode in HTX analyzers. A channel can still be
switch to the tunnel mode for legitimate cases (HTTP CONNECT or switching
protocols). So the function htx_adjust_conn_mode() is now useless.

This patch must be backported to 1.9. It is not strictly speaking required but
it will ease futur backports.