script/sign: remove needless IsSolvable() utility

It was used back when we didn't have a concept of descriptor. Now we
can check for solvability using descriptors.

src/wallet/scriptpubkeyman.cpp

@@ -1453,7 +1453,8 @@ void LegacyScriptPubKeyMan::LearnRelatedScripts(const CPubKey& key, OutputType t
         CTxDestination witdest = WitnessV0KeyHash(key.GetID());
         CScript witprog = GetScriptForDestination(witdest);
         // Make sure the resulting program is solvable.
-        assert(IsSolvable(*this, witprog));
+        const auto desc = InferDescriptor(witprog, *this);
+        assert(desc && desc->IsSolvable());
         AddCScript(witprog);
     }
 }