std/crypto/hmac: remove HmacBlake2s256 definition

HMAC is a generic construction, so we allow it to be instantiated
with any hash function.

In practice, HMAC is almost exclusively used with MD5, SHA1 and SHA2,
so it makes sense to define some shortcuts for them.

However, defining `HmacBlake2s256` is a bit weird (and why
specifically that one, and not other hash functions we also support?).
There would be nothing wrong with that construction, but it's not
used in any standard protocol and would be a curious choice.

BLAKE2 being a keyed hash function, it doesn't need HMAC to be used as
a MAC, so that also doesn't make it a good example of a possible hash
function for HMAC.

This commit doesn't remove the ability to use a Hmac(Blake2s256) type
if, for some reason, applications really need this, but it removes
HmacBlake2s256 as a constant.

lib/std/crypto/hmac.zig

@@ -18,10 +18,6 @@ pub const sha2 = struct {
     pub const HmacSha512 = Hmac(crypto.hash.sha2.Sha512);
 };
 
-pub const blake2 = struct {
-    pub const HmacBlake2s256 = Hmac(crypto.hash.blake2.Blake2s256);
-};
-
 pub fn Hmac(comptime Hash: type) type {
     return struct {
         const Self = @This();