Enhance PX1011 diagnostic's code fix by removing virtual modifiers from DAC field properties in DAC extension

[SENya1990]

Internal item number: ATR-631

The PX1011 diagnostic checks that DAC extension is marked as sealed to prevent creation of derived classes from it. Sometimes DAC extension contain DAC fields with virtual DAC field properties. Marking such extensions as sealed will lead to a compilation error.

We can improve the automated code fix so it won't break the compilation by removing virtual modifiers from DAC field properties in DAC extension.