Refactor IteRemoval preprocessing pass (#1793)

Refactor IteRemoval preprocessing pass (#1793)

This commit refactors the IteRemoval pass to follow the new format.
In addition to moving the code, this entails the following changes:

- The timer for the ITE removal is now called differently (the default
  timer of PreprocessingPass is used) and measures just the
  preprocessing pass without applySubstitutions(). It also measures the
  time used by both invocations of the ITE removal pass.
- Debug output will be slightly different because we now just rely on
  the default functionality of PreprocessingPass.
- d_iteRemover is now passed into the PreprocessingPassContext.
- AssertionPipeline now owns the d_iteSkolemMap, which makes it
  accessible by preprocessing passes. The idea behind this is that the
  preprocessing passes collect information in AssertionPipeline and
  d_iteSkolemMap fits that pattern.