all: bump go.mod version and drop compatibility shims

Also, remove the legacy import annotations.

Fixes golang/go#68147

Change-Id: Ibfcc9322f27224c0ba92ea42cd56912a7d8783fd
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/594256
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

salsa20/salsa/hsalsa20.go

@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // Package salsa provides low-level access to functions in the Salsa family.
-package salsa // import "golang.org/x/crypto/salsa20/salsa"
+package salsa
 
 import "math/bits"
 

salsa20/salsa20.go

@@ -19,7 +19,7 @@ This package also implements XSalsa20: a version of Salsa20 with a 24-byte
 nonce as specified in https://cr.yp.to/snuffle/xsalsa-20081128.pdf. Simply
 passing a 24-byte slice as the nonce triggers XSalsa20.
 */
-package salsa20 // import "golang.org/x/crypto/salsa20"
+package salsa20
 
 // TODO(agl): implement XORKeyStream12 and XORKeyStream8 - the reduced round variants of Salsa20.