Cleanup - Provider migration code

[Mzack9999]

Describe the bug
In early versions of subfinder the providers were defined within the config file. At some point they were moved to another file providers.yaml, and some glue logic was added to guarantee a smooth migration. Such code might no longer be needed since many versions were deployed, consider removing it:

• subfinder/v2/pkg/runner/options.go

  Line 82 in 0d90c63

  if fileutil.FileExists(defaultConfigLocation) && !fileutil.FileExists(defaultProviderConfigLocation) {

• subfinder/v2/pkg/runner/options.go

  Line 230 in 0d90c63

  func migrateToProviderConfig(defaultConfigLocation, defaultProviderLocation string) error {