Reverts unloading of digital persona

They don't properly cleanup (remove hooks they install) when we unload
them, leading to all sorts of crahes.

TBR as this is a revert.

BUG=291265
TEST=none
R=cpu@chromium.org
TBR=cpu@chromium.org

Review URL: https://codereview.chromium.org/26903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227962 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227962