...as ImpEditEngine::ImpRemoveChars (editeng/source/editeng/impedit2.cxx) calls

  pCurUndo->GetStr() += aStr;

Regression introduced with d5e11f5f "convert
editeng/source/editeng/editundo.hxx from String to OUString."

Change-Id: Iab3dd3b861bca715a90e89e3a63a6f298657367d
(cherry picked from commit e31205f3)
Reviewed-on: https://gerrit.libreoffice.org/19857Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>
(cherry picked from commit 5889d97f)