Remove reimplementation of PartDocument:close() in TextDocument

Summary: - with 1641d714d2ca a reimplementation of PartDocument:close() is not necessary any longer

Test Plan: - close KDevelop with a displayed kmarkdownwebview plugin

Reviewers: KDevelop, kossebau

Reviewed By: KDevelop, kossebau

Subscribers: kossebau, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D12527