all: delete redundant //go:debug gotypesalias=1 directives [generated]

The module's go directive is >= 1.23 now,
so these files no longer have any effect.

Also drop the outdated comment in go.mod.

For golang/go#69772.

[git-generate]
find . -type f -name gotypesalias.go -exec rm {} +
sed -i '' 's| // => default GODEBUG has gotypesalias=0||' go.mod

Change-Id: Id0fa54f89695991de6c8721aadecfd587826d158
Reviewed-on: https://go-review.googlesource.com/c/tools/+/650095
Reviewed-by: Cherry Mui <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Funda Secgin <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: dmitshur