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/checks.cpp

@@ -4,8 +4,8 @@
 
 #include <kernel/checks.h>
 
-#include <key.h>
 #include <random.h>
+#include <util/result.h>
 #include <util/translation.h>
 
 #include <memory>
@@ -14,10 +14,6 @@ namespace kernel {
 
 util::Result<void> SanityChecks(const Context&)
 {
-    if (!ECC_InitSanityCheck()) {
-        return util::Error{Untranslated("Elliptic curve cryptography sanity check failure. Aborting.")};
-    }
-
     if (!Random_SanityCheck()) {
         return util::Error{Untranslated("OS cryptographic RNG sanity check failure. Aborting.")};
     }