Clean generation technologies before redefining generation with sector method

[yerbol-akhmetov]

Describe the feature you'd like to see

There is no function to remove generators from power version in prepare_sector_network (maybe similar to patch_electricity_network of pypsa-eur). As a result, conventional generators converted to links+store+generator (eg. OCGT as defined in conventional_generation option of config) are still left, while links for OCGT are added. As a result, we have OCGT generator with carrier of OCGT, gas generator, gas store, and OCGT links (so double definition). That function can be optional.

It would be better to remove elec base generators and then keep only as links (with its store and generator). I will try to handle that and create a PR. Also it is important to properly transfer already insatlled capacities p_nom to links. I had an experience of adding existing capacities of conventional carriers to links (e.g. OCGT and so on).
What do you think? @davide-f @hazemakhalek

I am also pinning @martacki @danielelerede-oet to discuss it.