runtime: remove unused mutexevent linkname

CL 29650 added a sync.event linkname for mutexevent, but that does not
appear to have ever been used. This linkname isn't in the "hall of
shame", so we can just remove it.

Change-Id: I778b53ca089b5afda6c6074be9e43e3a6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/736220
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/mprof.go

@@ -834,7 +834,6 @@ func SetMutexProfileFraction(rate int) int {
 	return int(old)
 }
 
-//go:linkname mutexevent sync.event
 func mutexevent(cycles int64, skip int) {
 	if cycles < 0 {
 		cycles = 0