libc: cleanup deprecated heap-related Kconfigs

The following heap-related Kconfigs shared by the picolibc &
minimal libc have been deprecated for more than 2 releases,
remove them and update the Kconfigs accordingly:

- `CONFIG_MINIMAL_LIBC_MALLOC_ARENA_SIZE`
- `CONFIG_MINIMAL_LIBC_REALLOCARRAY`

Cleanup the handling to support the deprecated malloc arena
size configuration values.

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