keyids in "apt-key del" should be case-insensitive

keyids in "apt-key del" should be case-insensitive

gnupg is case-insensitive about keyids, so back then apt-key called it
directly any keyid was accepted, but now that we work more with the
keyid ourself we regressed to require uppercase keyids by accident.

This is also inconsistent with other apt-key commands which still use
gnupg directly. A single case-insensitive grep and we are fine again.

Closes: 781696