Minor cleanup

- Remove bbmask for meta-rust (maybe this belongs into another config
  file if needed, but I didn't need it when building with meta-rust or
  without)
- Remove rust-layer dependency - it's optional, so don't require it

conf/layer.conf

@@ -9,11 +9,7 @@ BBFILE_COLLECTIONS += "meta-slint"
 BBFILE_PATTERN_meta-slint = "^${LAYERDIR}/"
 BBFILE_PRIORITY_meta-slint = "6"
 
-LAYERDEPENDS_meta-slint = "core rust-layer clang-layer"
+LAYERDEPENDS_meta-slint = "core clang-layer"
 LAYERSERIES_COMPAT_meta-slint = "kirkstone langdale mickledore"
 
 RUST_PANIC_STRATEGY = "abort"
-
-# Don't use the poky provided one but meta-rust
-BBMASK += "poky/meta/recipes-devtools/rust"
-BBMASK += "poky/meta/recipes-devtools/cargo"