[release-branch.go1.12] crypto/rc4: remove false guarantees from Reset docs and deprecate it

Nothing in Go can truly guarantee a key will be gone from memory (see
#21865), so remove that claim. That makes Reset useless, because
unlike most Reset methods it doesn't restore the original value state,
so deprecate it.

Change-Id: I6bb0f7f94c7e6dd4c5ac19761bc8e5df1f9ec618
Reviewed-on: https://go-review.googlesource.com/c/162297


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit b35dacaa)
Reviewed-on: https://go-review.googlesource.com/c/163438