cmd/compile/internal/ssa: delete unused files

Change-Id: I5d640091375feb873517368ce05f0ba46c35714f
GitHub-Last-Rev: cdaf6ba
GitHub-Pull-Request: #44997
Reviewed-on: https://go-review.googlesource.com/c/go/+/301630
Reviewed-by: David Chase <[email protected]>
Trust: Keith Randall <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: cuiweixie