Remove `xAxisLabel` and `yAxisLabel` from `Layout`

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.

Start by reviewing the Layout API and the examples that use xAxisLabel or yAxisLabel. Confirm the removal decision first, then update every affected example and verify that the examples still compile and configure axis names through mapping or general axis settings.

Written by the indexing model from the issue text.