Nothing is actually scheduled, but is a first step towards using yarn.

As yarn is now actually used by SwiftShader, drop the BUILD_YARN build
flags, as failing to build yarn would result in linker errors.

Bug: b/139142453
Change-Id: Ibd9a69f72a248f58a62bb41eeb196c4647876e82
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35153Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>