cmd/link: don't suggest using nm to find -X name

It doesn't work if the package name includes a '.' or a non-ASCII
character (or '%', '"', or a control character).  See #16710 and CL 31970.

Update #18246.

Change-Id: I1487f462a3dc7b0016fce3aa1ea6239b226e6e39
Reviewed-on: https://go-review.googlesource.com/34791
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: ianlancetaylor

src/cmd/link/doc.go

@@ -41,8 +41,6 @@ Flags:
 		Set the value of the string variable in importpath named name to value.
 		Note that before Go 1.5 this option took two separate arguments.
 		Now it takes one argument split on the first = sign.
-		To discover which name to use, run `go tool nm PROGRAM`;
-		usable variables will be tagged "D" or "d".
 	-buildmode mode
 		Set build mode (default exe).
 	-cpuprofile file