Remove s2.h from s2cellid sources lists

Not all sources lists have been kept up to date with code
movement/removal and it seems to make some isolate steps in
cq sad for some CLs (mine).

Change-Id: I656a0a2f3aeaded7cace52ebc39cf80e1907163b
Reviewed-on: https://chromium-review.googlesource.com/c/1288872Reviewed-by: Jia Meng <jiameng@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#603437}

third_party/s2cellid/BUILD.gn

@@ -25,7 +25,6 @@ static_library("s2cellid") {
     "src/s2/r2rect.h",
     "src/s2/s1angle.cc",
     "src/s2/s1angle.h",
-    "src/s2/s2.h",
     "src/s2/s2cellid.cc",
     "src/s2/s2cellid.h",
     "src/s2/s2coords-internal.h",