[PDB] Remove truncate* (NFC)

- truncateQuotedNameFront: The last use was removed on Jul 10, 2017 in
  commit a9d944fd.

- truncateQuotedNameBack: The last use was removed on Mar 26, 2018 in
  commit 7b84b678.

- truncateStringMiddle: The last use was removed on Mar 26, 2018 in
  commit 7b84b678.

- truncateStringBack: The last use is in truncateQuotedNameBack being
  removed above.

- truncateStringFront: The last use is in truncateQuotedNameFront
  being removed above.