Remove non-default parameter settings in ACOPF models

[simonbowly]

We currently set these parameters for all OPF variants:

Non-default parameters:
MIPGap  0.001
OptimalityTol  0.001

In particular for ACrelax, these aren't relevant since the model is a QCP and is solved with barrier, so it's confusing to include them. For the exact models, I believe we loosened the MIPgap as achieving 1e-4 seemed unrealistic, but it may be better to leave parameters at defaults and remind the user in the docs to set a time limit?