Fix #3093 - remove declaration of unused function git_fetch__download_pack

Function was added in commit 2c982daa on October 5, 2011,
and removed in commit 41fb1ca0 on October 29, 2012.
Given the length of time it's gone unused, it's safe to remove now.

src/fetch.h

@@ -13,13 +13,6 @@ int git_fetch_negotiate(git_remote *remote, const git_fetch_options *opts);
 
 int git_fetch_download_pack(git_remote *remote, const git_remote_callbacks *callbacks);
 
-int git_fetch__download_pack(
-		git_transport *t,
-		git_repository *repo,
-		git_transfer_progress *stats,
-		git_transfer_progress_cb progress_cb,
-		void *progress_payload);
-
 int git_fetch_setup_walk(git_revwalk **out, git_repository *repo);
 
 #endif