Remove tts_core_notify_state_changed_async() to assure callback invocation

Remove tts_core_notify_state_changed_async() to assure callback invocation

- Issue:
Sometimes, if client calls tts_play() very quickly, then the first state changed callback which
shows the change from 'created' to 'ready' state is skipped.

- Solution:
This patch removes the tts_core_notify_state_changed_async() function in order to make sure that
the first state changed callback is called. tts_core_notify_state_changed_async() can not assure
the callback invocation of all state changed because state of client can be changed between timer
register and its actual invocation. And also, without this function, calback invocation does not
block the main loop because all the callers of this function is called by ipc message.
Thus, by this patch, client can assure that the state changed callback is always invoked when
the state is changed.

Change-Id: I2b333330d0c44caeb08973d41f800619458e292e
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>