Remove now-redundant file delete.
Remove now-redundant file delete.

After commit 365808e85, the entire sandbox directory is deleted,
but there's still a separate attempt to delete the temp file
created by the test asynchronously as well. If the file deletion
ends up happening after the sandbox is deleted, then we get a
spurious runtime error.

Fixes #212.

dart-lang/co19,dart-lang/co19,dart-lang/co19