Combine time and spatial derivative in spacetime_deriv_of_goth_g into…

[ncorsobh]

… a spacetime derivative in function input.

Proposed changes

Currently spacetime_deriv_of_goth_g takes the time and spatial derivatives of the lapse as inputs and combines them into a spacetime derivative. We now plan to do this earlier in the process, so this part of the function is being removed.

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