samples: wifi: shell: remove CONFIG_HEAP_MEM_POOL_SIZE for RW612

Remove CONFIG_HEAP_MEM_POOL_SIZE for RW612, it will use
HEAP_MEM_POOL_ADD_SIZE_ way to add all module's heap size
together, to better control heap size for different case.

Signed-off-by: Maochen Wang <maochen.wang@nxp.com>