[GeoMechanicsApplication] Clean up retention response

[avdg81]

📝 Description Cleaned up several things related to the calculation of the retention response.

🆕 Changelog

• When calculating the values at integration points, only call the relevant functions rather than CalculateRetentionResponse (which calculates all retention parameters). This saves us from calculating (and storing) unnecessary data.
• Consequently, it was no longer needed to calculate the effective saturation as part of CalculateRetentionResponse.
• Removed members FluidPressure and EffectiveSaturation from struct UPwSmallStrainElement::ElementVariables, since they had become redundant. Their usages have been replaced by adopting local variables. In a similar way, members FluidPressure and Density have been removed from struct SmallStrainUPwDiffOrderElement::ElementVariables.
• Removed two unused using statements.