In 336226c259 (packfile.h: drop extern from function declarations,
2019-04-05), `extern` was removed from function declarations because
it's redundant. However, in 8434e85d5f (repack: refactor pack deletion
for future use, 2019-06-10), an `extern` was mistakenly included.
Remove this spurious `extern`.
Signed-off-by: Denton Liu <liu.denton@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>