doc: remove shallow clone mention from go1.6.txt

It was reverted.

Change-Id: Ie30d8df9f2e5b14ff823fe81f5e538ee47064662
Reviewed-on: https://go-review.googlesource.com/17317Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.6.txt

@@ -4,7 +4,6 @@ cmd/dist: use clang on FreeBSD (https://golang.org/cl/16635)
 cmd/go: vendoring enabled by default (https://golang.org/cl/13967/)
 cmd/go: flags for tests must precede package name if present; also makes it easier to pass flags to test binaries (https://golang.org/cl/14826)
 cmd/go: add -msan option (https://golang.org/cl/16169)
-cmd/go: use shallow clones for new git checkouts (https://golang.org/cl/16360)
 cmd/compile: add -msan option (https://golang.org/cl/16160)
 cmd/link: add -msan option (https://golang.org/cl/16161)
 cmd/cgo: fix C.complexfloat and C.complexdouble and treat as separate types from complex64 and complex128 (https://golang.org/cl/17208)