Remove 'make update'.

In the CVS days this used be a wrapper around either CVS or CVSup and
used to support updating src, doc, and ports checkouts. With the move
to subversion this only supported updating src and was itself a
wrapper around 'svn update'. With Git, users are probably better off
using appropriate Git commands directly to update without needing an
explicit make target as a wrapper.

Reviewed by: bcr, imp, emaste
Differential Revision: https://reviews.freebsd.org/D30736