meta-firefox: remove dependency on rust-layer
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 <516df3ff67e1fa5d153800b15fb204c0a1a389dc@gmail.com>

OSSystems/meta-browser,OSSystems/meta-browser,OSSystems/meta-browser,OSSystems/meta-browser