Remove unnecessary sub thread checking logic

Remove unnecessary sub thread checking logic

- Issue:
Sometimes tts_destroy() is delayed without any reason.

- Solution:
This patch removes unncessary logic for checking active ecore sub
thread. In old version, tts.c module makes a sub thread for some reason
so the framework should check the active subthread for safe handle
destroy. However, since tizen 7.0, tts.c module does not make a sub
thread any more. And only the tts_core.c makes some sub thread and those
sub threads are completely controlled by the framework. So the checking
logic in tts.c is unncessary now.

Change-Id: I5f2ea8aa16f65e82e94332c046ae74ffba569f78
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>