Fix gn gen with all standalone targets disabled (#4249)
Fix gn gen with all standalone targets disabled (#4249)

* Fix gn gen with all standalone targets disabled

e.g. If configuring via

  gn gen out/nothing --args='target_os="all"  enable_default_builds=false'

The build fails with:

  ERROR at //BUILD.gn:201:26: Assignment had no effect.
    standalone_toolchain = "${chip_root}/config/standalone/toolchain:standalone"
                           ^----------------------------------------------------
  You set the variable "standalone_toolchain" here and it was unused before it went
  out of scope.

Inline ${standalone_toolchain} to fix this. It's a bit unfortunate that
uses in untaken branches trigger this diagnostic.

* Use not_needed() instead
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