lib: cpp: remove deprecated Kconfigs

These Kconfigs should have been deprecated for more than 2
releases, remove them:

- `CONFIG_CPP_MAIN`
- `CONFIG_CPLUSPLUS`
- `CONFIG_LIB_CPLUSPLUS`
- `CONFIG_EXCEPTIONS`
- `CONFIG_RTTI`

Signed-off-by: Yong Cong Sin <ycsin@meta.com>