remove electric-pair-delete-pair

I also noticed the function `electric-pair-backward-delete-char-untabify` was removed in the latest prelease builds of 24.4 but PR #235 actually breaks electric-pair compatibility. It makes every backspace run the delete pair function with no consideration of whether it is between a pair or not.

Luckily, the re-write of `elec-pair` in 24.4 is more unobtrusive so none of this is needed now for the desired backspace behavior.