Remove DPL String class and dependencies on it

Remove DPL String class and dependencies on it

Security-manager doesn't use DPL String, it was taken in as requirement
of DPL SQLConnection. The DPL String class introduces needless dependency
on libicu. Since our code doesn't operate on UTF-8 strings and doesn't
really need libicu, it's better to drop DPL String altogether.

Change-Id: Ia64a7e8ac8237642b0aae8b74bed28ddcaefe8c4
Signed-off-by: Rafal Krypa <r.krypa@samsung.com>