modules: tf-m: remove `S_NS_CONFIRMED_HEX_FILE`

`S_NS_CONFIRMED_HEX_FILE` was never generating a confirmed file, just
the same file contents as `S_NS_HEX_FILE`. Since no logic needs a
confirmed merge of `tfm_s.hex` and `zephyr.hex`, just remove the logic
instead of fixing it.

Signed-off-by: Jordan Yates <jordan@embeint.com>