DPL: work around for build warnings with -Werror

DPL: work around for build warnings with -Werror

DPL::SqlConnection::ExecCommand() is marked as deprecated to warn it's users.
It's not deprecated, but could cause SQL Injection. This method is also
used internally for transactions.
Such combination causes build warnings. But this repository is built with
-Werror flags and it breaks the build.

Since this method is not needed outside SqlConnection, move it to private
methods and remove the warning.

Change-Id: I088f4736ecca318613b897ef05b12af4cdc1f664
Signed-off-by: Rafal Krypa <r.krypa@samsung.com>