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/test/fuzz/script.cpp

@@ -89,7 +89,6 @@ FUZZ_TARGET_INIT(script, initialize_script)
     const FlatSigningProvider signing_provider;
     (void)InferDescriptor(script, signing_provider);
     (void)IsSegWitOutput(signing_provider, script);
-    (void)IsSolvable(signing_provider, script);
 
     (void)RecursiveDynamicUsage(script);