subtree: remove duplicate check

`cmd_add` starts with a check that the directory doesn't yet exist.
However, the `main` function performs the exact same check before
calling `cmd_add`.  So remove the check from `cmd_add`.

Signed-off-by: Luke Shumaker <lukeshu@datawire.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/git-subtree.sh

@@ -655,10 +655,6 @@ process_split_commit () {
 }
 
 cmd_add () {
-	if test -e "$dir"
-	then
-		die "'$dir' already exists.  Cannot add."
-	fi
 
 	ensure_clean