Remove the accounts window code from gtkaccount.[ch] as it has been replaced by PidginAccountManager

Testing Done:
Verified that the account manager and editor continued to function as it was previously.

Note, that before we rewrote the account manager, editors were tracked and removed when an account was removed, but that was broken in the rewrite and requires a bit more work that I wanted to put into this review request right now.

Reviewed at https://reviews.imfreedom.org/r/1439/