framework: Remove signal type __FWK_ID_TYPE_SIGNAL

This change removes unused types
 __FWK_ID_TYPE_SIGNAL and 'struct signal'
which are not needed as it's use was removed in the
below commit(part of light event patchset)

f74c34fc framework, dvfs: Remove signals.

Change-Id: I46b0c90ffc55893df5c2cebf461637ba91a8f3ce
Signed-off-by: Girish Pathak <girish.pathak@arm.com>