Remove assignee_id from Bitbutcket imports

Not sure if we're supposed to support setting assignees at some point
and why we're setting it to nil in the first place.

lib/gitlab/bitbucket_import/importer.rb

@@ -182,7 +182,6 @@ module Gitlab
             target_branch_sha: target_branch_sha,
             state: pull_request.state,
             author_id: gitlab_user_id(project, pull_request.author),
-            assignee_id: nil,
             created_at: pull_request.created_at,
             updated_at: pull_request.updated_at
           )

lib/gitlab/bitbucket_server_import/importer.rb

@@ -211,7 +211,6 @@ module Gitlab
           target_branch_sha: pull_request.target_branch_sha,
           state_id: MergeRequest.available_states[pull_request.state],
           author_id: author_id,
-          assignee_id: nil,
           created_at: pull_request.created_at,
           updated_at: pull_request.updated_at
         }