meta-firefox: remove dependency on rust-layer

* for honister it's possible to use rust from oe-core, for older releases
  build without meta-rust will fail, but the error message should be relatively
  clear what needs to be added:
  meta-firefox/recipes-devtools/cbindgen/cbindgen_0.8.7.bb:3: Could not inherit file classes/cargo.bbclass

Signed-off-by: Martin Jansa <[email protected]>

Author: shr-project

meta-firefox/conf/layer.conf

@@ -115,4 +115,4 @@ BBFILE_PRIORITY_firefox-browser-layer = "7"
 LAYERVERSION_firefox-browser-layer = "1"
 LAYERSERIES_COMPAT_firefox-browser-layer = "dunfell gatesgarth hardknott honister"
 
-LAYERDEPENDS_firefox-browser-layer = "clang-layer core openembedded-layer rust-layer"
+LAYERDEPENDS_firefox-browser-layer = "clang-layer core openembedded-layer"