libdpkg: Remove unused showcopyright prototype

This was forgotten in commit 06b64be7ba505c51f8673e7ba323629e9c60574d.

lib/dpkg/myopt.h

@@ -49,9 +49,6 @@ void myfileopt(const char* fn, const struct cmdinfo* cmdinfos);
 void myopt(const char *const **argvp, const struct cmdinfo *cmdinfos);
 void loadcfgfile(const char *prog, const struct cmdinfo *cmdinfos);
 
-/* Utility functions. */
-void showcopyright(const struct cmdinfo *cip, const char *value) DPKG_ATTR_NORET;
-
 DPKG_END_DECLS
 
 #endif /* MYOPT_H */