Remove "Manage accounts" from PeopleActivity

The string "menu_accounts" is not in use any more, but it's
in ContactsCommon. I don't want to create a 2-line CL to
delete it, and will delete it when working on the next CL in
ContactsCommon.

Bug 27687799
Bug 28230930

Change-Id: I1e478e1e87068e69be94ca24408deee5d49acb30