core: Remove copy assignment operator from CoreIdentity

core: Remove copy assignment operator from CoreIdentity

The operator is no longer needed due to the switch from QList
to std::vector that was done in the previous commit.

This avoids a warning in GCC 9:

    src/core/coreidentity.cpp: In member function ‘CoreIdentity& CoreIdentity::operator=(const CoreIdentity&)’:
    src/core/coreidentity.cpp:91:23: warning: implicitly-declared ‘Identity& Identity::operator=(const Identity&)’ is deprecated [-Wdeprecated-copy]

The warning is caused by relying on the implicit copy assignment
operator in the base class, which is no longer required by the
standard since C++11, and may be removed in the future.