Some constants were added to flate that seem to be an experimental
attempt at increasing the window size. However, according to RFC1951,
the largest window size is 32KiB, so these constants are non-standard.
Delete them.

Fixes #18458

Change-Id: Ia94989637ca031a56bce2548624fa48044caa7b9
Reviewed-on: https://go-review.googlesource.com/60490
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>