No need to release resources in destructor of CPDF_ShadingObject

CPDF_DocPageData::~CPDF_DocPageData() will force to release all resources, so no need to do it here, which can result in heap-use-after-free trouble.

BUG=408164
R=jun_fang@foxitsoftware.com, tsepez@chromium.org

Review URL: https://codereview.chromium.org/513063003