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>

src/sync/map.go

@@ -12,10 +12,9 @@ import (
 // Map is a concurrent map with amortized-constant-time loads, stores, and deletes.
 // It is safe for multiple goroutines to call a Map's methods concurrently.
 //
-// Map is designed to reduce cache contention in the Go standard library.
-// It is optimized for use in concurrent loops with keys that are stable
-// over time, and either few steady-state stores, or stores localized to
-// one goroutine per key.
+// It is optimized for use in concurrent loops with keys that are
+// stable over time, and either few steady-state stores, or stores
+// localized to one goroutine per key.
 //
 // For use cases that do not share these attributes, it will likely have
 // comparable or worse performance and worse type safety than an ordinary