restrict fit_xy() from working for censored regression models

[hfrick]

Closes #442

Models with the new mode "censored regression" can't use fit_xy(), they should use fit().

surv_reg models have also been forced to use the formula interface fit(). The check for that has been moved into fit_xy() from check_xy_interface() which checks the details of x and y. The corresponding tests for check_xy_interface() were removed. The tests for an error when using fit_xy() with a surv_reg model are untouched in test_sruv_reg_survreg.R and test_surv_reg_flexsurv.R.

The corresponding tests for models for censored regression are to be added when we have the corresponding model specs in parsnip (#444).

[topepo]

We can add test cases for this when we move the model definitions to parsnip from censored.

[hfrick]

PR #449 adds the model spec for survival_reg which uses the new mode and includes a test on fit_xy() throwing an error when used with this mode.

[github-actions]

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