gcrypt: Remove limited crypto plugin

This has not seen any significant changes for years.  So it lacks support
for modern algorithms and would require quite some work for an overhaul.
Given that we support several other crypto backends, let's just remove
this to reduce the maintenance burden.

The test scenarios and other references are also removed.

scripts/dh_speed.sh

@@ -11,15 +11,12 @@ function modptest {
 }
 
 echo "testing gmp"
-# gmp needs an RNG plugin, pick gcrypt
-modptest "gmp gcrypt"
+# gmp needs an RNG plugin, pick random
+modptest "gmp random"
 
 echo "testing curve25519"
-# curve25519 needs an RNG plugin, pick gcrypt
-$DIR/dh_speed "curve25519 gcrypt" 300 curve25519 curve25519 | tail -n 1
-
-echo "testing gcrypt"
-modptest "gcrypt"
+# curve25519 needs an RNG plugin, pick random
+$DIR/dh_speed "curve25519 random" 300 curve25519 curve25519 | tail -n 1
 
 echo "testing openssl"
 modptest "openssl"