promisor-remote.h: drop extern from function declaration

promisor-remote.h: drop extern from function declaration

During the creation of this file, each time a new function declaration
was introduced, it included an `extern`. However, starting from
554544276a (*.[ch]: remove extern from function declarations using
spatch, 2019-04-29), we've been actively trying to prevent externs from
being used in function declarations because they're unnecessary.

Remove these spurious `extern`s.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>