Revert "cmd/internal/goobj: add index to symbol name for indexed symbols"

This reverts CL 229246.

For new indexed object files, in CL 229246 we added symbol index
to tools (nm, objdump) output. This affects external tools that
parse those outputs. And the added index doesn't look very nice.
In this release we take it out. For future releases we may
introduce a flag to tools (nm, objdump) and optionally dump the
symbol index.

For refererenced (not defined) indexed symbols, currently the
symbol is still referenced only by index, not by name. The next
CL will make the object file self-contained, so tools can dump
the symbol names properly (as before).

For #38875.

Change-Id: I07375e85a8e826e15c82fa452d11f0eaf8535a00
Reviewed-on: https://go-review.googlesource.com/c/go/+/236167


Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>