tree-wide: Remove dead code

This code seems to not be used anywhere at all, so let's drop it.

Follow up for ba8d00e859
Follow up for 85686b37b0
Follow up for aff853f8ea
Follow up for 36dad381a2

src/basic/unit-def.h

@@ -306,8 +306,6 @@ const char* unit_dbus_interface_from_name(const char *name);
 const char* unit_type_to_string(UnitType i) _const_;
 UnitType unit_type_from_string(const char *s) _pure_;
 
-const char* unit_type_to_capitalized_string(UnitType t);
-
 const char* unit_load_state_to_string(UnitLoadState i) _const_;
 UnitLoadState unit_load_state_from_string(const char *s) _pure_;