Made sed expression GNU and BSD compatible.

A recent change in pass.zsh-completion broke autocompletion when using
BSD sed. I’ve made the relevant sed expression compatible with GNU and
BSD sed.

Previous change with regression:
http://lists.zx2c4.com/pipermail/password-store/2014-April/000773.html

http://git.zx2c4.com/password-store/commit/src/completion/pass.zsh-completion?id=f82e9d6cf3bc3a12bdfce89bf319d76f79e66efc

Optional force does not make sense for non-interactive remove function.

Reported-by: Lukas Fleischer <info@cryptocrack.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>