Remove obsolete asserts for QGradient::ObjectBoundingMode

If our SVG parser creates objects with a gradient in
QGradient::ObjectBoundingMode, then we shouldn't assert
when encountering those.

This fixes

50:libs-kritaflake-TestSvgParser
51:libs-kritaflake-TestSvgParserCloned
52:libs-kritaflake-TestSvgParserRoundTrip