Remove inadvertently added outgoing/packfile.h

This empty file was inadvertently introduced in commit 4f39cd8 ("pack:
move pack name-related functions", 2017-08-23). Remove this file.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jonathantanmy