Remove duplicated logic int stop behavior

Remove duplicated logic int stop behavior

- Issue:
In __stop_by_silence() function, there is duplicated logic.

- Solution:
In previous code, __stop_by_silence() function send message to client
according to the return value of sttd_server_stop(). However,
sttd_server_stop() is already send message when it is return STATE DONE
value. Thus, the bahvior on __stop_by_silence() is actually unvaluable.
Thus, this patch removes that code to reduce duplicated logic.

Change-Id: I61948b1ad6fe6d243b1240fb8ae484639b072e0e
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>