crypto/elliptic: remove deprecation markers

crypto/elliptic: remove deprecation markers

These should be deprecated, but per go.dev/wiki/Deprecated,
that should only happen two releases after the replacement is
available (so Go 1.22).

The deprecation of this package was part of the discussion
of proposal #52221. All that remains is waiting for the new
package to be widely available.

Change-Id: I580a4af6514eb77d7ec31b443d07259a4a2cf030
Reviewed-on: https://go-review.googlesource.com/c/go/+/453256
Reviewed-by: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>