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.h

@@ -5,10 +5,6 @@
 #ifndef BITCOIN_KERNEL_CONTEXT_H
 #define BITCOIN_KERNEL_CONTEXT_H
 
-#include <util/signalinterrupt.h>
-
-#include <memory>
-
 namespace kernel {
 //! Context struct holding the kernel library's logically global state, and
 //! passed to external libbitcoin_kernel functions which need access to this
@@ -19,7 +15,6 @@ namespace kernel {
 //! should be stored to std::unique_ptr members pointing to opaque types.
 struct Context {
     Context();
-    ~Context();
 };
 } // namespace kernel