Remove deprecated keep-max-bpc-unchanged debug flag

It's been a stub for two releases now.

docs/wiki/Configuration:-Debug-Options.md

@@ -324,27 +324,6 @@ debug {
 }
 ```
 
-### `keep-max-bpc-unchanged`
-
-<sup>Since: 25.08</sup>
-
-When connecting monitors, niri sets their max bpc to 8 in order to reduce display bandwidth and to potentially allow more monitors to be connected at once.
-Restricting bpc to 8 is not a problem since we don't support HDR or color management yet and can't really make use of higher bpc.
-
-Apparently, setting max bpc to 8 breaks some displays driven by AMDGPU.
-If this happens to you, set this debug flag, which will prevent niri from changing max bpc.
-AMDGPU bug report: https://gitlab.freedesktop.org/drm/amd/-/issues/4487.
-
-<sup>Since: 25.11</sup>
-This setting is deprecated and does nothing: niri no longer sets max bpc.
-The old niri behavior with this setting enabled matches the new behavior.
-
-```kdl
-debug {
-    keep-max-bpc-unchanged
-}
-```
-
 ### Key Bindings
 
 These are not debug options, but rather key bindings.

niri-config/src/debug.rs

@@ -10,7 +10,6 @@ pub struct Debug {
     pub enable_overlay_planes: bool,
     pub disable_cursor_plane: bool,
     pub disable_direct_scanout: bool,
-    pub keep_max_bpc_unchanged: bool,
     pub restrict_primary_scanout_to_matching_format: bool,
     pub force_disable_connectors_on_resume: bool,
     pub render_drm_device: Option<PathBuf>,
@@ -42,8 +41,6 @@ pub struct DebugPart {
     #[knuffel(child)]
     pub disable_direct_scanout: Option<Flag>,
     #[knuffel(child)]
-    pub keep_max_bpc_unchanged: Option<Flag>,
-    #[knuffel(child)]
     pub restrict_primary_scanout_to_matching_format: Option<Flag>,
     #[knuffel(child)]
     pub force_disable_connectors_on_resume: Option<Flag>,
@@ -82,7 +79,6 @@ impl MergeWith<DebugPart> for Debug {
             enable_overlay_planes,
             disable_cursor_plane,
             disable_direct_scanout,
-            keep_max_bpc_unchanged,
             restrict_primary_scanout_to_matching_format,
             force_disable_connectors_on_resume,
             force_pipewire_invalid_modifier,

niri-config/src/lib.rs

@@ -2252,7 +2252,6 @@ mod tests {
                 enable_overlay_planes: false,
                 disable_cursor_plane: false,
                 disable_direct_scanout: false,
-                keep_max_bpc_unchanged: false,
                 restrict_primary_scanout_to_matching_format: false,
                 force_disable_connectors_on_resume: false,
                 render_drm_device: Some(