Revert "arch: arm: cortex_m: Add API for scb save and restore"

This reverts commit a90a47b1.

This commit was written with CMSIS 5 in mind, where some Cortex-M cores
have "SHP" in the SCB_Type, and some have "SHPR". This is not correct as
Zephyr is *supposed* to be using CMSIS 6 for Cortex-M... but CI actually
picks up CMSIS 5 instead (it includes both with CMSIS 5 taking priority).

The end result is that Zephyr's CI builds this happily but it causes build
failures on downstream users (e.g., example-application).

Revert the commit now, as it is not used yet by anyone. The revised version
using only "SHPR" shall be reintroduced once the CI issue has been fixed.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>