Fix client releasing code simpler

Fix client releasing code simpler

- Issue:
tts_config_mgr_unset_callback should be invoked for all clients when
they are released.

- Solution:
This patch fixes the __tts_config_release_client()'s behavior. Through
this patch, __tts_config_release_client() will invoke
tts_config_mgr_unset_callback() to reset all callbacks in client
structure.
And plus, this patch changes the name and type of these functions. These
two functions are only invoked in this file. This means that those
functions are private. So, this patch makes these functions as static
and changes the name of these functions which avoids the reserved
identifiers.

Change-Id: I2d20ca73595eacca5a37d8e84822a8d4e0602df7
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>