Revert "workaround on windows for llvm6 missing advapi32.lib in llvm-config"

This reverts commit eaac218d59.

This is fixed now in llvm6 rc3

build.zig

@@ -168,10 +168,6 @@ fn findLLVM(b: &Builder, llvm_config_exe: []const u8) !LibraryDep {
                 }
             }
         }
-        // workaround for advapi32.lib missing from output
-        if (builtin.os == builtin.Os.windows) {
-            try result.system_libs.append("advapi32.lib");
-        }
     }
     {
         var it = mem.split(includes_output, " \r\n");