crypto/tls: remove tlsrsakex GODEBUG setting

Updates #75316

Change-Id: I6eb8482505a83b8b63edcb7d443e227a6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/777381
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

defaults.go

@@ -66,7 +66,6 @@ func defaultSupportedSignatureAlgorithms() []SignatureScheme {
 	}
 }
 
-//var tlsrsakex = godebug.New("tlsrsakex")
 //var tls3des = godebug.New("tls3des")
 
 func supportedCipherSuites(aesGCMPreferred bool) []uint16 {
@@ -81,7 +80,6 @@ func defaultCipherSuites(aesGCMPreferred bool) []uint16 {
 	cipherSuites := supportedCipherSuites(aesGCMPreferred)
 	return slices.DeleteFunc(cipherSuites, func(c uint16) bool {
 		return disabledCipherSuites[c] ||
-			rsaKexCiphers[c] ||
 			tdesCiphers[c]
 	})
 }