Add back deprecation warning for Opt_WarnCompatUnqualifiedImports

@monoidal writes

If we're abandoning this migration, then for backwards compatibility let's at least keep the flag in a vestigal form for a few releases:
  Opt_WarnCompatUnqualifiedImports ->
    -> depWarnSpec x "This warning no longer does anything; see GHC #24904"

I failed to take advice of his comment and just removed the warning. We should add back this deprecation before the 9.12 release.

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