internal/runtime/maps: remove redundant package docs

internal/runtime/maps: remove redundant package docs

This CL removes redundant package docs comments under different files
as all of them will be shown in pkgsite.

As of now, package docs 'Package maps implements Go's builtin map type'
appears 3 times in pkgsite.

See: https://pkg.go.dev/internal/runtime/maps

Change-Id: I2f0696dd4f860afea5cc346532bca59135bec798
Reviewed-on: https://go-review.googlesource.com/c/go/+/702135
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>