Remove unused AppKeepAliveService

For app_shell (go/appshell) we need to remove the circular dependencies from src/apps back to src/chrome. AppKeepAliveService has dependencies on Chrome but is unused, so let's delete it instead of fixing it.

BUG=326040
TEST=compiles
TBR=erg@chromium.org for deleting an unused include from chrome/browser/profiles/chrome_browser_main_extra_parts_profiles.cc

Review URL: https://codereview.chromium.org/172313004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253695 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253695