sql: Remove references to USE_SYSTEM_SQLITE.

USE_SYSTEM_SQLITE used to be supported on iOS, and now is a no-op.
This CL removes the references to it from //sql, to avoid confusing
readers.

The CL also breaks down an unnecessarily large unit test in
//sql/connection_unittest.cc that happend to be near the changes in this
CL.

Change-Id: I47c324300b54362529364a5ef26ee45b45fe3b76
Reviewed-on: https://chromium-review.googlesource.com/1137921
Reviewed-by: Chris Mumford <[email protected]>
Reviewed-by: Daniel Murphy <[email protected]>
Commit-Queue: Daniel Murphy <[email protected]>
Cr-Commit-Position: refs/heads/master@{#576213}
diff --git a/sql/connection.h b/sql/connection.h
index d45db948..57fc5453 100644
--- a/sql/connection.h
+++ b/sql/connection.h
@@ -396,9 +396,7 @@
   //
   // On the SQLite version shipped with Chrome (3.21+, Oct 2017), databases can
   // be attached while a transaction is opened. However, these databases cannot
-  // be detached until the transaction is committed or aborted. On iOS, the
-  // built-in SQLite might not be older than 3.21. In that case, attaching a
-  // database while a transaction is open results in a error.
+  // be detached until the transaction is committed or aborted.
   bool AttachDatabase(const base::FilePath& other_db_path,
                       const char* attachment_point);
   bool DetachDatabase(const char* attachment_point);