(dev/core#1387) FiveTwentyThree - Drop civicrm_domain.config_backend if it snuck in

(dev/core#1387) FiveTwentyThree - Drop civicrm_domain.config_backend if it snuck in

This column was dropped in 4.7.alpha1, but it was still created on new installs, so

* Sites originating with 4.7 - 5.22 should have an empty copy of the column today
  (because it was defined in xml schema - but never used by live code).
* Sites originating with <4.7 (and subsequently upgraded) should not have the column today
  (because FourSeven.php previously dropped it).
* Sites originating with <4.7 (and not yet upgraded) should have the column with some data
  (because FourSeven.php hasn't run yet).
* It is unexpected that any site currently on 4.7-5.22 would have any data in that column.

Note: `dropColumn` has a built-in guard so it only drops the column if it
exists.