Remove dumb-allocator

- Remove the actual code
- Remove mentions of it from iallocator.rst, and use hail instead
- Also remove mentions of "etch-image" and use "debootstrap+default"
- Mention htools as the reference implementation in iallocator.rst

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>