remove virtual from operator=
remove virtual from operator=

Completely pointless, no inherited class overrides it, in fact
nothing even seems to use it, and it generates a warning
with -Woverloaded-virtual.

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core