libkmod: remove unused inline functions

libkmod: remove unused inline functions

libkmod/libkmod-list.c:39:33: warning: unused function 'list_node_next' [-Wunused-function]
static inline struct list_node *list_node_next(const struct list_node *node)
                                ^
libkmod/libkmod-list.c:47:33: warning: unused function 'list_node_prev' [-Wunused-function]
static inline struct list_node *list_node_prev(const struct list_node *node)
                                ^

It doesn't really matter in the end result since the compiler won't
generate any code for it. But let's keep it clean. It wasn't needed
until now, so probably it won't be anymore.