libstore: remove HttpBinaryCacheStore::getFileContents

the default implementation using getFile does exactly the same thing,
unless the thing being downloaded goes away immediately before we try
to resume a partial download. that's best treated as a different kind
of error though because the target did exist the first time we asked.

Change-Id: Ic0c046bdcd9d9751c6ba78167c68a38d0d2739e9

lix/libstore/http-binary-cache-store.cc

@@ -160,20 +160,6 @@ protected:
         }
     }
 
-    std::optional<std::string> getFileContents(const std::string & path) override
-    {
-        checkEnabled();
-
-        try {
-            return getFileTransfer()->download(makeURI(path)).second->drain();
-        } catch (FileTransferError & e) {
-            if (e.error == FileTransfer::NotFound || e.error == FileTransfer::Forbidden)
-                return {};
-            maybeDisable();
-            throw;
-        }
-    }
-
     /**
      * This isn't actually necessary read only. We support "upsert" now, so we
      * have a notion of authentication via HTTP POST/PUT.