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/env-util.h

@@ -82,6 +82,4 @@ int getenv_path_list(const char *name, char ***ret_paths);
 
 int getenv_steal_erase(const char *name, char **ret);
 
-int set_full_environment(char **env);
-
 int setenvf(const char *name, bool overwrite, const char *valuef, ...) _printf_(3,4);