libexpr: DrvInfo: remove unused bad-citizen constructor

DrvInfo's constructor that only takes `EvalState` leaves everything else
empty; a DrvInfo which has no iota of information about the derivation
it represents is not useful, and was not used anywhere.

Change-Id: Ic4d93a08cb2748b8cef9a61e41e70404834b23f9

src/libexpr/get-drvs.hh

@@ -45,7 +45,6 @@ public:
      */
     std::string attrPath;
 
-    DrvInfo(EvalState & state) : state(&state) { };
     DrvInfo(EvalState & state, std::string attrPath, Bindings * attrs);
     DrvInfo(EvalState & state, ref<Store> store, const std::string & drvPathWithOutputs);