* nNextElementNumber is unused since d57dfa29
  "Improvement of adding and removing color charts".

* There appears to be no compelling reason to store sDefaultNamePre-/Postfix as
  members (but which are not even copied by the copy ctor).  Just compute them
  whenever they are needed in getDefaultName (which is understood to not be a
  hot code path).

* With those changes, the user-provided default and copy ctor can be left
  implicitly-declared, which avoids -Wdeprecated-copy (because the copy
  assignment op was implicitly-declared while the copy ctor was not) with GCC
  trunk towards GCC 9.

Change-Id: I2bfa5f19c967e24407bf5b79adaf171200822f45
Reviewed-on: https://gerrit.libreoffice.org/56666
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>