Remove xwalk-pkg-helper

BUG=XWALK-2609

xwalk-pkg-helper was introduced to do privileged work during installation process.

It lost it's purpose after removing suid from it.
No extra privileges / extra binary is needed anymore.
About this class. I would like to leave this as it is for now. It is encapsulation for pkgmgr API and signal as you. So it have some point for now at least. Installation/Uninstalaltion function are huge enough in xwalk_package_installer.cc