common: Remove the copy assignment operator from DccConfig

Since C++11 the standard no longer mandates the existence of an
implicit copy constructor if there is a user-supplied copy assignment
operator, and GCC 9 warns about that:

src/qtui/settingspages/dccsettingspage.cpp: In member function ‘virtual void DccSettingsPage::load()’:
src/qtui/settingspages/dccsettingspage.cpp:91:71: warning: implicitly-declared ‘DccConfig::DccConfig(const DccConfig&)’ is deprecated [-Wdeprecated-copy]

Remove the copy assignment operator altogether and rely on the
implicitly generated one, which does the same thing.