Amalgamate the 3 step and 6 step flow code in `gauge_wilson_flow.cu` and `gauge_wilson_flow.cuh`

[cpviolator]

As part of #1555 a new 4th order (6 step) flow algorithm was introduced alongside the original 3 step one. These two Runge-Kutta integrators can be merged with the correct use of coefficients for the 3 step version. This will eliminate the specific computeW1Step, computeW2Step, and computeVTStep function in gauge_wilson_flow.cuh and both integrators can be performed using computeWStep only.

This should be addressed after both #1555 and #1526 have been merged.