Remove partial classes/modules from CONTRIBUTING?

[srittau]

We still have a section about "Incomplete stubs" in CONTRIBUTING. I believe we should remove this as nowadays stubs are generally generated using stubgen, so this shouldn't be an issue anymore. I don't want to encourage new incomplete stubs, and it makes the CONTRIBUTING file a longer read.

We only should leave the sentence about unannotated arguments and return values.

[ramvikrams]

Done with the changes

[Avasam]

With this, should ignore_missing_stub default to false? Since "we don't want to encourage new incomplete stubs". It's mainly needed for stubs that are already present and incomplete, or would result in an allowlist that's too complex. (the create_baseline_stubs script does set it to false, I'm thinking about running stubtest with it false by default)