drm/xe: Drop unused IS_PLATFORM_STEP() and IS_SUBPLATFORM_STEP()

drm/xe: Drop unused IS_PLATFORM_STEP() and IS_SUBPLATFORM_STEP()

The macros IS_PLATFORM_STEP() and IS_SUBPLATFORM_STEP() are unused since
commit 87c299fa3a97 ("drm/xe/guc: Port Wa_14014475959 to xe_wa and fix
it") and commit 63bbd800ff01 ("drm/xe/guc: Port
Wa_22012727170/Wa_22012727685 to xe_wa"), respectively, and we can drop
them now.  Furthermore, in upcoming changes we will add logic to read
platform-level step information from PCI RevID and keeping those macros
around would potentially cause confusion.

v2:
  - Cite commits that made the macros unused. (Matt)

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patch.msgid.link/20260309-extra-nvl-p-enabling-patches-v5-2-be9c902ee34e@intel.com
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>