blockimgdiff.py: Clean up stash id computation in BBOTA v3+.

Only BBOTA v2 needs to maintain a pool of available 'stash slot id'.
BBOTA v3+ uses the hash of the stashed blocks as the slot id, which
doesn't need the id pool anymore.

Bug: 33694544
Test: Generate v2 and v4 incrementals w/ and w/o the CL. They produce
      the same packages respectively.
Change-Id: I8121af5b6b1bee98c3639d54a00b06fd12e378e8