github_import: Fix reference to removed `remote_name` parameter

We have recently removed support for the `remote_name` parameter, but
the GitHub importer was still referring to it in one location. Fix this
by removing the parameter.

lib/gitlab/github_import/importer/repository_importer.rb

@@ -50,7 +50,7 @@ module Gitlab
           project.ensure_repository
 
           refmap = Gitlab::GithubImport.refmap
-          project.repository.fetch_as_mirror(project.import_url, refmap: refmap, forced: true, remote_name: 'github')
+          project.repository.fetch_as_mirror(project.import_url, refmap: refmap, forced: true)
 
           project.change_head(default_branch) if default_branch
 

spec/lib/gitlab/github_import/importer/repository_importer_spec.rb

@@ -202,7 +202,7 @@ RSpec.describe Gitlab::GithubImport::Importer::RepositoryImporter do
 
       expect(repository)
         .to receive(:fetch_as_mirror)
-        .with(project.import_url, refmap: Gitlab::GithubImport.refmap, forced: true, remote_name: 'github')
+        .with(project.import_url, refmap: Gitlab::GithubImport.refmap, forced: true)
 
       service = double
       expect(Repositories::HousekeepingService)