mfd: twl-core: Remove unused add_child() and add_numbered_child() functions

Clang with W=1 reports:

  drivers/mfd/twl-core.c:654:30: error: unused function 'add_child' [-Werror,-Wunused-function]
  static inline struct device *add_child(unsigned mod_no, const char *name,
                               ^
add_numbered_child() and its only caller add_child() are not used, so remove them.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230322115838.2569414-1-trix@redhat.com