Deprecate `quantile_reg()`

[dajmcdon]

This is now supported directly: linear_reg() |> set_mode("quantile regression", quantile_levels = c(0.2, 0.5, 0.8)).

• We can remove make_quantile_reg.R
• Adjust arx_fcast_epi_workflow() to process the quantile levels correctly.

Aside: That's a terrible function name.