[Support] Remove TaskQueue

TaskQueue was added several years ago as part of D48240.

There are currently no uses of this class anywhere in LLVM and I don't see
any patch that plans to use this class, so it doesn't seem useful to keep
compiling and testing this class at the moment.

The code itself is fine, so if we actually end up having a use for this code,
then I think it's perfectly fine to just re-commit this class then.

Differential revision: https://reviews.llvm.org/D86338