Remove text settings that make translation difficult

Simple text settings should not be in Ubercart's UI; instead users should be encouraged to use translation or String Overrides to alter short text strings. These settings make it almost impossible to translate these strings into multiple languages unless special support is added for Variable module; it is easier to remove the strings from the UI instead. This applies to at least the following:

• "Continue shopping" text
• PayPal "submit order" button text
• 2Checkout "submit order" button text