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.

INSTALL

@@ -59,8 +59,6 @@ Contents
         https://botan.randombit.net
       * The GNU Multiprecision Arithmetic Library (GMP, libgmp)
         https://gmplib.org
-      * The GNU Cryptographic Library (libgcrypt)
-        https://www.gnupg.org
 
     If no other options are specified during ./configure libgmp will be used.