drop unused task.h header
drop unused task.h header

This code doesn't use this header anymore, and newer libbase doesn't
provide it, so stop trying to include it.

BUG=chromium-os:26646
TEST=`emerge-x86-alex chromeos-login` still works

Change-Id: I291e8ee443c3569b9bfc24e4064dd401d480445b
Reviewed-on: https://gerrit.chromium.org/gerrit/18496
Reviewed-by: Chris Masone <98e3d65c6b0e9e2391eb4c2e8b1e572b77e544ed@chromium.org>
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager