rust-dirstatemap: remove `set_dirstate_entry`/`set_entry` methods
These methods were needed crutches before the Rust implementation caught up
to Python. Calling `set_entry` (whether from Python or Rust) was dangerous
since it didn't update any of the counters of the DirstateMap data structure,
while having no real way of knowing when to use it "correctly" except it you
were one of the 3 people who looked deep enough into the soul of this code.
Differential Revision: https://phab.mercurial-scm.org/D12519
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