notes remove: --ignore-missing

notes remove: --ignore-missing

Depending on the application, it is not necessarily an error for an object
to lack a note, especially if the only thing the caller wants to make sure
is that notes are cleared for an object.  By passing this option from the
command line, the "git notes remove" command considers it a success if the
object did not have any note to begin with.

Signed-off-by: Junio C Hamano <gitster@pobox.com>