intel/compiler: Delete abs/neg handling in fsign code

intel/compiler: Delete abs/neg handling in fsign code

This should have gone away when removing source modifiers.  They won't
be set any longer, so this is simply dead code.

Fixes: b7c47c4f7cf ("intel/compiler: Drop nir_lower_to_source_mods() and related handling.")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4691>