...as it does the same as the copy assignment op.  And both of those are
apparently already only called with SolarMutex locked, so no need to lock it
again.  So the copy assignment op (as well as the other special memeber
functions) can be left implicitly-declared.  (And some uses of the original
takeOver can be optimized to use move assignment.)

Change-Id: I279a5e3ee85ff2342d6ef5f672108a0712fe116d
Reviewed-on: https://gerrit.libreoffice.org/72831
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>