package_parser: remove redundant pkg size param

Since the package is passed as a std::vector, that already describes the
size of the package.

Both 'update_manager.cpp' and 'item_update_manager.cpp' pass the size of
the vector containing the package.

Change-Id: I45b381a5ad5b4488a3db540f59f04f15402d264e
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>