Remove references from fixed output derivation ab syntax

In other words, use a plain `ContentAddress` not
`ContentAddressWithReferences` for `DerivationOutput::CAFixed`.

Supporting fixed output derivations with (fixed) references would be a
cool feature, but it is out of scope at this moment.

src/libstore/content-address.cc

@@ -196,6 +196,11 @@ const Hash & ContentAddress::getHash() const
     }, raw);
 }
 
+std::string ContentAddress::printMethodAlgo() const {
+    return getMethod().renderPrefix()
+        + printHashType(getHash().type);
+}
+
 bool StoreReferences::empty() const
 {
     return !self && others.empty();
@@ -271,9 +276,4 @@ Hash ContentAddressWithReferences::getHash() const
     }, raw);
 }
 
-std::string ContentAddressWithReferences::printMethodAlgo() const {
-    return getMethod().renderPrefix()
-        + printHashType(getHash().type);
-}
-
 }