drm/i915: remove unused config DRM_I915_UNSTABLE

Essentially this is a revert of commit d9d54a53 ("drm/i915: Put
future HW and their uAPIs under STAGING & BROKEN").

We currently have no users for this config option. The last one was
removed in commit 8c26491f ("drm/i915: Kill the fake lmem
support"). Drop it altogether; it's easy enough to resurrect if need
arises.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230327105330.312131-2-jani.nikula@intel.com

drivers/gpu/drm/i915/Kconfig

@@ -164,11 +164,5 @@ menu "drm/i915 Profile Guided Optimisation"
 	source "drivers/gpu/drm/i915/Kconfig.profile"
 endmenu
 
-menu "drm/i915 Unstable Evolution"
-	visible if EXPERT && STAGING && BROKEN
-	depends on DRM_I915
-	source "drivers/gpu/drm/i915/Kconfig.unstable"
-endmenu
-
 config DRM_I915_GVT
 	bool

drivers/gpu/drm/i915/Kconfig.unstable

-# SPDX-License-Identifier: GPL-2.0-only
-config DRM_I915_UNSTABLE
-	bool "Enable unstable API for early prototype development"
-	depends on EXPERT
-	depends on STAGING
-	depends on BROKEN # should never be enabled by distros!
-	# We use the dependency on !COMPILE_TEST to not be enabled in
-	# allmodconfig or allyesconfig configurations
-	depends on !COMPILE_TEST
-	default n
-	help
-	  Enable prototype uAPI under general discussion before they are
-	  finalized. Such prototypes may be withdrawn or substantially
-	  changed before release. They are only enabled here so that a wide
-	  number of interested parties (userspace driver developers) can
-	  verify that the uAPI meet their expectations. These uAPI should
-	  never be used in production.
-
-	  Recommended for driver developers _only_.
-
-	  If in the slightest bit of doubt, say "N".