Revert to use of isset($_POST[]) for submit buttons

Fix the unfortunately incomplete revert of post_key_exists().
See #699 for the known issues, mainly nonfunctional delete `<button>`s.
Also reported as #755.

post_key_exists() was intended to replace, unify and name the necessary
usage of isset($_POST[]).  This attempt drops the replace and name part :).

I assumed post_key_exists() was already in the right places, and ran a
global search+replace on it.

git checkout -b tmp
git revert be42cff
git diff -w > revert.patch
git checkout develop
git branch -D tmp

patch -p1 < revert.patch #+1 fix of rejected hunk

git rm application/core/MY_Input.php

find -name *.php -exec sed -e "s/\\\\\$this->input->post_key_exists('\([^']*\)')/isset(\\\\\$_POST['\1'])/g" -i \{\} \;

find -name *.php -exec sed -e "s/\$this->input->post_key_exists('\([^']*\)')/isset(\$_POST['\1'])/g" -i \{\} \;

git grep post_key_exists
Merge branch 'feature/revert-post-key-exists' into develop