Don't set default visibility

This way the visiblility will default to the one in user preferences. By
default this is 'public'.

toot/api.py

@@ -183,7 +183,7 @@ def post_status(
     app,
     user,
     status,
-    visibility='public',
+    visibility=None,
     media_ids=None,
     sensitive=False,
     spoiler_text=None,

toot/cli/post.py

@@ -40,7 +40,6 @@ from toot.utils.datetime import parse_datetime
     "--visibility", "-v",
     help="Post visibility",
     type=click.Choice(VISIBILITY_CHOICES),
-    default="public",
 )
 @click.option(
     "--sensitive", "-s",
@@ -119,7 +118,7 @@ def post(
     media: Tuple[str],
     descriptions: Tuple[str],
     thumbnails: Tuple[str],
-    visibility: str,
+    visibility: Optional[str],
     sensitive: bool,
     spoiler_text: Optional[str],
     reply_to: Optional[str],