Do not suggest deprecated flags

When you misspell the name of a flag, GHC outputs a nice list of similar flags that do exist:

$ ghc-8.0.1 -W-redundant-constraints
ghc: unrecognised flag: -W-redundant-constraints
did you mean one of:
  -Wredundant-constraints
  -Wno-redundant-constraints
  -fwarn-redundant-constraints

But since the -fwarn flags are deprecated, they should not be suggested. Neither should any of the other deprecated flags.

For a newcomer: the code is compiler/main/DynFlags.hs.

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Trac field	Value
Version	7.10.3
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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