SfxPoolItem has the curious design of non-deleted copy ctor and deleted copy
assignment op.  Many derived classes then provide a non-deleted assignment op,
apparently more or less on an as-needed basis.  But some of those user-provided
assignment ops are actually unused (and their presence causes
-Werror=deprecated-copy with GCC trunk towards GCC 9 when the---implicitly-
defined---copy ctor of the derived class is used), so remove them.  In some
cases that would still cause -Werror=deprected-copy because of a user-declared
dtor, so in those cases explicitly default all the copy/move functions (some of
which will then be implicitly defined as deleted).

Change-Id: If0d9f1a9f3954263a39ffd27ba895d6202afa307
Reviewed-on: https://gerrit.libreoffice.org/58133
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>