Drop the -frtti flag from tink_config

This is already achieved by removing //build/config:no_rtti config.

Change-Id: Id5dd6ac5d054a959ce05b1d29148e5ef7d1c6a5c

BUILD.gn

@@ -5,7 +5,6 @@
 config("tink_config") {
   cflags = [
     "-Wno-ignored-qualifiers",
-    "-frtti",
   ]
   include_dirs = [
     "$target_gen_dir",