arch: cortex_m/r: kconfig: Remove unused LDREX_STREX_AVAILABLE symbol

Existed already in commit 8ddf82cf ("First commit"). Has never been
used.

Found with a script.

Also remove some pointless menus that have no visible symbols in them.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>