Profile replacement does not always work

Our docs say:
> Once a profile is activated, the specified `build`, `test` and `deploy` configuration in it will replace the `build`, `test` and `deploy` sections declared in the main section of `skaffold.yaml`.

This implies that a pipeline like the following:
```
build:
artifacts:
- image: example

profiles:
- name: example
build: {}
```

applying the `example` profile should replace the `build:` sections, such that there are no artifacts to build. But it does not.

We should either fix this, or change the documentation to reflect the reality that we ~merge~ overlay the paths found in the profile:
https://github.com/GoogleContainerTools/skaffold/blob/d9b397ecb0d075aee50554c8c7b054b85f1958f7/pkg/skaffold/schema/profiles.go#L206-L210