cmd/compile: remove spurious ir.Dump

This ir.Dump call is a debugging artifact introduced in
golang.org/cl/274103, which should never be printed for valid,
non-generic code, but evidently can now sometimes appear due to how
the parser handles invalid syntax.

The parser should probably not recognize "x[2]" as a type expression
in non-generics mode, but also probably we shouldn't try noding after
reporting syntax errors. Either way, this diagnostic has outlived its
usefulness, and noder's days are numbered anyway, so we might as well
just remove it to save end users any confusion.

Updates #46558.

Change-Id: Ib68502ef834d610b883c2f2bb11d9b385bc66e37
Reviewed-on: https://go-review.googlesource.com/c/go/+/324991


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>