cronet: Fix BUILD.gn to follow aosp/2374766

aosp/2374766 removed some targets from the BUILD.gn but there are some
targets that depend on the removed targets, which makes gn gen fail.
This CL fixes clean up targets to fix this issue.

Test: gn gen
Change-Id: Ie0bf5ac609e0550c4f6370fc1d65fdde8b57c5bb
2 files changed