Bug 2053553 Remove unused WebRender debug preferences. r=emilio,layout-reviewers

The following WebRender debug preferences are no longer in use. They are no longer in the GFX_WEBRENDER_DEBUG
flag table in gfx/thebes/gfxPlatform.cpp.

modules/libpref/init/all.js:
	- gfx.webrender.debug.alpha-primitives
	- gfx.webrender.debug.slow-frame-indicator

modules/libpref/init/StaticPrefList.yaml:
	- gfx.webrender.debug.slow-cpu-frame-threshold

Differential Revision: https://phabricator.services.mozilla.com/D322553

Author: BlessedOneKobo

modules/libpref/init/StaticPrefList.yaml

@@ -8362,11 +8362,6 @@
   value: false
   mirror: always
 
-- name: gfx.webrender.debug.slow-cpu-frame-threshold
-  type: AtomicFloat
-  value: 10.0
-  mirror: always
-
 - name: gfx.webrender.late-scenebuild-threshold
   type: RelaxedAtomicInt32
   value: 4

modules/libpref/init/all.js

@@ -409,15 +409,13 @@ pref("gfx.webrender.debug.texture-cache", false);
 pref("gfx.webrender.debug.texture-cache.clear-evicted", true);
 pref("gfx.webrender.debug.render-targets", false);
 pref("gfx.webrender.debug.gpu-cache", false);
-pref("gfx.webrender.debug.alpha-primitives", false);
 pref("gfx.webrender.debug.profiler", false);
 pref("gfx.webrender.debug.gpu-time-queries", false);
 pref("gfx.webrender.debug.gpu-sample-queries", false);
 pref("gfx.webrender.debug.disable-batching", false);
 pref("gfx.webrender.debug.epochs", false);
 pref("gfx.webrender.debug.echo-driver-messages", false);
 pref("gfx.webrender.debug.show-overdraw", false);
-pref("gfx.webrender.debug.slow-frame-indicator", false);
 pref("gfx.webrender.debug.picture-caching", false);
 pref("gfx.webrender.debug.picture-borders", false);
 pref("gfx.webrender.debug.force-picture-invalidation", false);