roots: remove duplicate default scan mode setting

Set the constructor's default scan mode to an invalid mode, so if we
change the default, we don't have to update two places.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees.h

@@ -604,7 +604,7 @@ public:
 	void set_workaround_btrfs_send(bool do_avoid);
 
 private:
-	ScanMode m_scan_mode = SCAN_MODE_INDEPENDENT;
+	ScanMode m_scan_mode = SCAN_MODE_COUNT; // must be set
 	static string scan_mode_ntoa(ScanMode new_mode);
 
 };