arch: x86: segmentation: Remove unused _far_call() and _far_jump()

These functions were not used throughout the Zephyr code base, and
as such has been removed.  They can be reinstated if there's a need,
but will need to be adapted to use retpolines when CONFIG_RETPOLINE
is set.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>