[dev.link] cmd/link: remove symbol.FuncInfo

It is no longer used. The only remaining use is in generating
Plan 9 debug info, which is already not supported.

Change-Id: Ia023d6f2fa7d57b97ba861ce464e2eec8ac2d1f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/228142


Reviewed-by: Than McIntosh <thanm@google.com>