Delete canRotate

[samreid]

canRotate seems unused. Can we delete it?

[AgustinVallejo]

Did this but would benefit from another pair of eyes. The code removal was pretty straightforward, but I had to change MassIO because it had a stateKey associated with canRotate. Added the following migration rule, and it seems to be congrulatory, but again, assigning back to @samreid for review:

    new ChangeStateKeyIOType( 'MassIO', 'tag', 'MassTagIO', mass => {
      delete mass.canRotate;
    } )

[zepumph]

Looks great, thanks.