remove unneeded check using non-API R_UnboundValue

Author: Enchufa2

inst/include/Rcpp/api/meat/Rcpp_eval.h

@@ -56,10 +56,6 @@ inline SEXP Rcpp_eval(SEXP expr, SEXP env) {
     // 'identity' function used to capture errors, interrupts
     Shield<SEXP> identity(Rf_findFun(::Rf_install("identity"), R_BaseNamespace));
 
-    if (identity == R_UnboundValue) {
-        stop("Failed to find 'base::identity()'");
-    }
-
     // define the evalq call -- the actual R evaluation we want to execute
     Shield<SEXP> evalqCall(Rf_lang3(::Rf_install("evalq"), expr, env));