Remove support for SAGE_UPGRADING
Trac #17072: Always rebuild depending packages (SAGE_UPGRADING=yes)

Remove the `SAGE_UPGRADING` environment variable and always build as if
it was set to `yes`.

URL: http://trac.sagemath.org/17072
Reported by: jdemeyer
Ticket author(s): Jeroen Demeyer
Reviewer(s): Volker Braun