[NFC][RemoveDIs] Remove conditional compilation for RemoveDIs (#81149)

A colleague observes that switching the default value of
LLVM_EXPERIMENTAL_DEBUGINFO_ITERATORS to "On" hasn't flipped the value
in their CMakeCache.txt. This probably means that everyone with an
existing build tree is going to not have support built in, meaning
everyone in LLVM would need to clean+rebuild their worktree when we flip
the switch on... which doesn't sound good.

So instead, just delete the flag and everything it does, making everyone
build and run ~400 lit tests in RemoveDIs mode. None of the buildbots
have had trouble with this, so it Should Be Fine (TM).

(Sending for review as this is changing various comments, and touches
several different areas -- I don't want to get too punchy).