Fixes #1974: Correct C-h behavior to ensure electric-pair deletes adjacent pairs

[jamescherti]

When using electric-pair-mode, the expected behavior when pressing C-h near a pair of adjacent delimiters (e.g., () [] {} "") is that both the opening and closing delimiters should be deleted together if they were inserted as a pair.

This pull request fixes #1974.

This pull request corrects C-h behavior to ensure electric-pair deletes adjacent pairs. It calls the same function as DEL (code 127) in electric-pair-mode-map:

electric-pair-mode-map is a variable defined in ‘elec-pair.el’.

Its value is
(keymap 
 (127 menu-item "" electric-pair-delete-pair :filter
  #f(compiled-function (cmd) #<bytecode 0x115035a9952592b3>)))

This pull request could be improved. Please don't hesitate to comment it if you have any suggestions.