runtime: during map delete, update entries after new last element

When we delete an element, and it was the last element in the bucket,
update the slots between the new last element and the old last element
with the marker that says "no more elements beyond here".

Change-Id: I8efeeddf4c9b9fc491c678f84220a5a5094c9c76
Reviewed-on: https://go-review.googlesource.com/c/142438


Reviewed-by: Matthew Dempsky <mdempsky@google.com>