Remove unneeded CustomViewPager class

Originally this hacky class has been introduced by I0824f82c30 to
address bug: 11721001.  Because the bug: 11721001 has been addressed
by I950f13ec40 and etc., this class should be removed.

Bug: 13665882
Change-Id: Iabdd369e4dddb0188534289eea1269fb196895eb