kernel: Remove key module from kernel library

The key module's functionality is not used by the kernel library, but
currently kernel users are still required to initialize the key module's
`secp256k1_context_sign` global as part of the `kernel::Context` through
`ECC_Start`.

src/kernel/context.cpp

@@ -5,9 +5,7 @@
 #include <kernel/context.h>
 
 #include <crypto/sha256.h>
-#include <key.h>
 #include <logging.h>
-#include <pubkey.h>
 #include <random.h>
 
 #include <string>
@@ -19,12 +17,7 @@ Context::Context()
     std::string sha256_algo = SHA256AutoDetect();
     LogPrintf("Using the '%s' SHA256 implementation\n", sha256_algo);
     RandomInit();
-    ECC_Start();
 }
 
-Context::~Context()
-{
-    ECC_Stop();
-}
 
 } // namespace kernel