Remove reference to BoringSSL in preparation for removing BoringSSL itself from third_party.

--
MOS_MIGRATED_REVID=121937338
diff --git a/third_party/BUILD b/third_party/BUILD
index 6d7724b..f163a54 100644
--- a/third_party/BUILD
+++ b/third_party/BUILD
@@ -3,7 +3,6 @@
 filegroup(
     name = "srcs",
     srcs = glob(["**"]) + [
-        "//third_party/boringssl:srcs",
         "//third_party/css/bootstrap:srcs",
         "//third_party/css/font_awesome:srcs",
         "//third_party/ijar:srcs",