arm: cmsis: Remove unused code from scb/scs layers

In preperation for removing the scb/scs layers and using CMSIS directly
lets remove all the _Scb* and _Scs* functions that are not currently
used.

Jira: ZEP-1568

Change-Id: If4641fb9a6de616b4b8793d4678aaaed48e794bc
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>