Assure that client always registers the callback in sync prepare

Assure that client always registers the callback in sync prepare

- Issue:
Sometimes, the message receive callback registration is skipped even if
the connection is normal.

- Solution:
The client lib always checks the 'register_callback_invoked' flag, but
this flag may not be set properly some cases even if client tries to
prepare synchronously. Thus, this patch removes the flag checking code
from synchronous prepare logic. The synchronous prepare should insure
the invocation of all protocol, so through this patch, the all step of
preparation will be invoked.

Change-Id: If89f4065f139ec032bedddebf6a9927c74f100e7
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>