build: ignore unittest targets in dependency removal

since the unittests are linked into the test binary in ways
which are beyond the tool and accidentally removed targets will
not be run, decreasing test coverage.

Applied to
  modules/congestion_controller/BUILD.gn
as an example.

Bug: webrtc:467294026
Change-Id: Ic44a8eb3e2250c857bcea0145bf6f2d4ac995113
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/451881
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Jeremy Leconte <jleconte@google.com>
Cr-Commit-Position: refs/heads/main@{#46987}