configure: Remove unused CONFIG_SIGEV_THREAD_ID switch

The CONFIG_SIGEV_THREAD_ID switch is unused since the related code
has been removed by commit 6d327171
("aio / timers: Remove alarm timers"), so it can safely be removed
nowadays.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1465571084-19885-1-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>