GenericStringRef::operator= does not compile

[yachoor]

GenericStringRef::operator= tries to assign values to constant members, failing to compile:

rapidjson::GenericStringRef<char> a("");
rapidjson::GenericStringRef<char> b("");
a = b;

generates errors:
error: assignment of read-only member 'rapidjson::GenericStringRef::s'
error: assignment of read-only member 'rapidjson::GenericStringRef::length'

Unfortunately one of the compilers I use is overeager with member functions of templates and fails compilation even if it is never called :(
Since it doesn't compile anyways and compiler-generated default one will fail the same I think it can be safely deleted.

[miloyip]

Thank you