Remove deepOverride

It's unused, and also a bad idea: because it recursively recomputes
every function argument and there is no sharing, you can get an
exponential (?) blowup in evaluation time. For example, evaluating
‘linuxPackages.kernel’ takes 0.09s and ~13 MiB, but evaluating
‘linuxPackages.kernel.deepOverride {}’ takes 3.6s and ~305 MiB.