cmd/link: delete stale deadcode reference

Back when the linker did code generation after dead code elimination,
it had to know that references to runtime.read_tls_fallback could be
generated at code generation time (and never appear before that). Now
that code generation is done by the compiler, the references to
runtime.read_tls_fallback are obvious in the relocations, so the
linker no longer needs special knowledge of this symbol.

Change-Id: I9813a8478e85a6a13470b2d0528db53fd33fcfdf
Reviewed-on: https://go-review.googlesource.com/c/154601


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>