MINOR: mux_h2: do not try to remove front conn from idle trees

MINOR: mux_h2: do not try to remove front conn from idle trees

In h2_process there was two parts where the connection was removed from
the idle trees, without first checking if the connection is a backend
side.

This should not produce a crash as the node is properly zeroed on
conn_init. However, it is better to explicit the test as it is done on
all other places. Besides it will be mandatory if the node part is
dynamically allocated only for backend connections.