sync: delete a sentence from the Map docs

sync: delete a sentence from the Map docs

From Josh's comments on https://golang.org/cl/50310

Once I removed the "from the Go standard library" bit, the beginning
wasn't worth keeping. It also wasn't clear whether what it meant by
"cache contention". Processor caches, or user-level caches built with
sync.Map? It didn't seem worth clarifying and didn't convey any useful
information, so deleted.

Change-Id: Id1d76105a3081d0855f6a64540700932bb83d98e
Reviewed-on: https://go-review.googlesource.com/50632
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>