Remove xAxisLabel and yAxisLabel from Layout

[AndreiKingsley]

They were added there due to the fact that this was the way it was done in the Lets-Plot API. However, the question of whether they are needed in Kandy is still unresolved.
These labels can be configured in the corresponding mapping or general axis settings:

plot {
   line {
      x(col) {
         axis.name = "..."
      }
   }
   x.axis.name = "..."
}

If we decide to remove it, we have a lot of examples that use this API that need to be updated.