Remove SymlinkOutputs

symlink_outputs was added so bazel could run ninja files, but we
abanoned that approach in roboleaf, and then roboleaf was cancelled
entirely. Remove this feature so we're more compatible with upstream
ninja / n2.

Bug: 160568334
Test: Presubmits
Change-Id: Ic368c48dd01b68e51c471c3fe90d0c02c55956e9