cmd/link: remove name expansion logic

cmd/link: remove name expansion logic

Now both the compiler and the assembler require the -p flag and
emit full package path in symbol names, we no longer need to do
the name expansion in the linker. Delete it.

Change-Id: I771d4d97987a0a17414881b52806d600ef4cc351
Reviewed-on: https://go-review.googlesource.com/c/go/+/404300
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>