[clang][dataflow] Remove buggy assertion. (#67311)

The assertion fails on the test
TransferTest.EvaluateBlockWithUnreachablePreds
(which I think, ironically, was introuced in the same patch as the
assertion).

This just wasn't obvious because the assertion is inside an `LLVM_DEBUG`
block
and is thus only executed if the command-line flag `-debug` is passed.
We don't
have any CI builds that do this, so it's almost guaranteed that
assertions like
this will start failing over time (if they ever passed in the first
place --
which I'm not sure about here).

It's not clear to me whether there's _some_ assertion we might be able
to make
here -- I've looked at this for a while but haven't been able to come up
with
anything obvious. For the time being, I think it's best to simply delete
the
assertion.