bash completion: remove unused function _git_diff_tree

completion for git diff-tree was removed in 5cfb4fe5Signed-off-by: NLee Marlow <lee.marlow@gmail.com>
Acked-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -721,11 +721,6 @@ _git_diff ()
 	__git_complete_file
 }
 
-_git_diff_tree ()
-{
-	__gitcomp "$(__git_refs)"
-}
-
 _git_fetch ()
 {
 	local cur="${COMP_WORDS[COMP_CWORD]}"