Remove THD::dup_field and instead check if there are fewer columns
in the write set than in the list of inserted fields. Use this info
to report ER_FIELD_SPECIFIED_TWICE for all sets of duplicate
columns during INSERT.

This achieves two things:
- Reduces THD state.
- Allows all sets of duplicate columns to be reported as errors,
  not just the first.

Change-Id: I26d512436e462d986284e8f5775375b148891bc2