cmd/link: eliminate markextra

cmd/link: eliminate markextra

This appears to be leftover from when instruction selection happened
in the linker. Many of the morestackX functions listed don't even
exist anymore.

Now that we select instructions within the compiler and assembler,
normal deadcode elimination mechanisms should suffice for these
symbols.

Change-Id: I2cb1e435101392e7c983957c4acfbbcc87a5ca7d
Reviewed-on: https://go-review.googlesource.com/38077
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>