build: Get rid of CLIENT_DATE

Putting the build date in the executable is a practice that has no place
in these days, now that deterministic building is increasingly common.

Continues #7732 which did this for the GUI.

src/clientversion.h

@@ -59,7 +59,6 @@ static const int CLIENT_VERSION =
 
 extern const std::string CLIENT_NAME;
 extern const std::string CLIENT_BUILD;
-extern const std::string CLIENT_DATE;
 
 
 std::string FormatFullVersion();