instance: drop an unneeded "want wait() to terminate" reset assignment

It's uncertain why srd_inst_decode() which internally gets called by the
public srd_session_send() routine used to clear the want_wait_terminate
flag. This should be cleared upon decoder instance creation and state
reset, gets raised upon termination request, but shall not get cleared
in other spots, especially not upon the availability of new input data
while the stream shall be considered in the "about to terminate, skip
all subsequent execution" state.