exec: simplify destruction of the phys map

Do not bother visiting the radix tree when an address space is destroyed.
After the previous patch, this has become a pointless exercise.  When
called from address_space_destroy_dispatch, all you're doing is zeroing
out a structure that will be freed as soon as you come back.  When called
from mem_begin, when phys_page_set_level will call phys_map_node_alloc the
radix tree's array will be zeroed too.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>