Remove unused indexes from `deployments` table

Problem

According to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22799#note_115119853, It seems we don't use the following indexes in deployments table.

  add_index "deployments", ["environment_id", "id"], name: "index_deployments_on_environment_id_and_id", using: :btree
  add_index "deployments", ["project_id", "iid"], name: "index_deployments_on_project_id_and_iid", unique: true, using: :btree
  add_index "deployments", ["created_at"], name: "index_deployments_on_created_at", using: :btree
  add_index "deployments", ["environment_id", "status"], name: "index_deployments_on_environment_id_and_status", using: :btree
  add_index "deployments", ["project_id", "status"], name: "index_deployments_on_project_id_and_status", using: :btree
  add_index "deployments", ["environment_id", "iid", "project_id"], name: "index_deployments_on_environment_id_and_iid_and_project_id", using: :btree

We should make sure if those are indexes are truly unused, and if it's the case, we should drop the index.