b-cs: Define G_TRANSLATION_FUN in bitcoinkernel.cpp

[META] This is done in preparation for extracting libbitcoinkernel in a
       following commit. It seems logical that generally users of a
       library shouldn't need to export its own symbols to use the
       library.

src/bitcoin-chainstate.cpp

@@ -28,8 +28,6 @@
 #include <functional>
 #include <iosfwd>
 
-const std::function<std::string(const char*)> G_TRANSLATION_FUN = nullptr;
-
 int main(int argc, char* argv[])
 {
     // SETUP: Argument parsing and handling