Remove dead code from PrerenderManager.

FindPrerenderManagerUsingRenderProcessId isn't called anywhere.

BUG=none

Review URL: https://codereview.chromium.org/154553002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249429 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249429