kernel: remove unused _THREAD_TIMEOUT_INIT and _THREAD_ERRNO_INIT

_THREAD_TIMEOUT_INIT() has been replaced by _nano_timeout_thread_init(),
so it can be removed.

_THREAD_ERRNO_INIT() is defined, but never used. Ben suspects that this
is a bug, and that there should be some code that calls it.

Jira: ZEP-1326
Change-Id: I476c316b80e9f34d1ed61971229ed9afafc80d8a
Signed-off-by: Anas Nashif <anas.nashif@intel.com>