replace: remove signature when using --graft

replace: remove signature when using --graft

It could be misleading to keep a signature in a
replacement commit, so let's remove it.

Note that there should probably be a way to sign
the replacement commit created when using --graft,
but this can be dealt with in another commit or
patch series.

Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>