Sync Failed VSP Tickets does require VSP_LISTING even when disabled in Privacy/Security Settings

[karamble]

Lately https://api.decred.org/?c=vsp was malformed json / UTF-8 headers

The result were INVALID_VSPS errors in decrediton GUI.

Even when the VSP Listing was disabled in the privacy/security settings this error occured which means it still is requesting the API when using the 'Sync Failed VSP Tickets' function in the staking tab.

If the user has disabled the VSP Listing in his Privacy Settings and specifies a VSP URL manually the 'Sync Failed VSP Tickets' function should not send out requests to the API endpoint and instead only try to use the manually specified VSP.