west: tf-m: Remove tf-m from Zephyr LTS

Zephyr mbedTLS was updated to 2.28.x which is a LTS release and
address several vulnerabilities affecting 2.26 (version that used to be
used on Zephyr LTS).

Unfortunately this mbedTLS version is not compatible with TF-M and
backporting mbedTLS fixes was not a viable solution. Due this problem
we are removing TF-M module from Zephyr's LTS. One still can go and add
it to this manifest if needed, but this is no longer "officially"
supported.

More information in:
https://github.com/zephyrproject-rtos/zephyr/issues/56071
https://github.com/zephyrproject-rtos/zephyr/pull/54084



Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>