[dev.typeparams] cmd/compile: remove CallExpr.PreserveClosure

This flag is only needed to prevent the directClosureCall optimization
in walkCall, when called for walkGoDefer. But walkGoDefer don't need
to call walkCall: at this point in the compile, the call expression
isn't a real call anymore.

Instead, we just need to walkExpr on the function expression.

Change-Id: I8a5176cfe1bff53700cbd21ed1b479ebd9a839ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/330271


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Matthew Dempsky <mdempsky@google.com>