Prevent error message from showing when s:dummy_sign_id is not set

I'm calling `:GitGutterEnable` and `:GitGutterDisable` as part of another toggle function that also shows/hides line numbers. An issue came up when it would call `:GitGutterDisable` on a file that wasn't in a git repository.

`E121: Undefined variable: s:dummy_sign_id`
`E15: Invalid expression: s:dummy_sign_id "file=" . s:file()`

This change checks to see if `s:dummy_sign_id` exists before it removes it.