Fix bad RPATH in db8 tests.

:Release Notes:
Remove using of CMAKE_INSTALL_RPATH.

:Detailed Notes:
CMAKE_INSTALL_RPATH doesn't fix the RPATH issue so we need to remove it.

:Testing Performed:
Manual testing.

:QA Notes:

:Issues Addressed:
[BHV-1435] Bad RPATH in db8-tests

Open-webOS-DCO-1.0-Signed-off-by: Andrii Motsok <andrii.motsok@lge.com>

Change-Id: I5fa7a0c3def26969c04097aa19567eed75a4e796
Reviewed-on: https://g2g.palm.com/4849
Reviewed-by: DCO Verification
Reviewed-by: Andrii Motsok <andrii.motsok@lge.com>
Reviewed-by: Maksym Sditanov <maxim.sditanov@lge.com>
Tested-by: Maksym Sditanov <maxim.sditanov@lge.com>