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

Makefile

@@ -24,7 +24,6 @@
 # kernel-toolchain    - Builds the subset of world necessary to build a kernel
 # kernel-toolchains   - Build kernel-toolchain for all universe targets.
 # doxygen             - Build API documentation of the kernel, needs doxygen.
-# update              - Convenient way to update your source tree(s).
 # checkworld          - Run test suite on installed world.
 # check-old           - List obsolete directories/files/libraries.
 # check-old-dirs      - List obsolete directories.
@@ -155,7 +154,7 @@ TGTS=	all all-man buildenv buildenvvars buildkernel buildworld \
 	installkernel.debug packagekernel packageworld \
 	reinstallkernel reinstallkernel.debug \
 	installworld kernel-toolchain libraries maninstall \
-	obj objlink showconfig tags toolchain update \
+	obj objlink showconfig tags toolchain \
 	makeman sysent \
 	_worldtmp _legacy _bootstrap-tools _cleanobj _obj \
 	_build-tools _build-metadata _cross-tools _includes _libraries \