Automatically delete unprotected Velocity namespace

This means that unless the namespace is "protected" with a call to VelocityEngine#startedUsingMacroNamespace it will be removed at the end of evaluate when all you want is just to execute some isolated velocity code.

This allows to get rid of a lot of annoying calls to startedUsingMacroNamespace/stoppedUsingMacroNamespace and any code that really require to share macros between several velocity contents is already calling startedUsingMacroNamespace/stoppedUsingMacroNamespace (mostly to make sure they will share the same namespace and that l will be removed at the end).