Delete `virtual` keyword for overriding methods

[hollasch]

Just had this come up at work: the virtual keyword is redundant with override or final. Either of the last two imply that the function is virtual.

Hence a common recommendation is that the first declaration of a virtual function get the virtual keyword, and that all derived classes use either override or final and do not use the virtual keyword. Makes sense, and reduces unnecessary keyword clutter. I think we should adopt this convention.

[trevordblack]

I approve

[hollasch]

Done.