checkout: remove duplicate code

Both commit a7256debd4 ("checkout.txt: note about losing staged
changes with --merge", 2019-03-19) from nd/checkout-m-doc-update and
commit 6eff409e8a ("checkout: prevent losing staged changes with
--merge", 2019-03-22) from nd/checkout-m were included in git.git
despite the fact that the latter was meant to be v2 of the former.
The merge of these two topics resulted in a redundant chunk of code;
remove it.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -745,13 +745,6 @@ static int merge_working_tree(const struct checkout_opts *opts,
 				      "the following files:\n%s"), sb.buf);
 			strbuf_release(&sb);
 
-			if (repo_index_has_changes(the_repository,
-						   get_commit_tree(old_branch_info->commit),
-						   &sb))
-				warning(_("staged changes in the following files may be lost: %s"),
-					sb.buf);
-			strbuf_release(&sb);
-
 			/* Do more real merge */
 
 			/*