Remove imgcaptcha, this code is unused in any of our forks and needs work to get here back.

Revert "Merge branch 'imgcaptcha' of https://github.com/asiekierka/Tinyboard"

This reverts commit 49afb685c8e658c0f36b8171e176363538eb9419, reversing
changes made to 2451a0f29c08e23ff36ef113082c112224f7e576.

Conflicts:

	js/imgcaptcha.js