Remove the builder-files option

You can now include files via the "builders" option, using the syntax
"@<filename>". Having only one option makes it easier to override
builders completely.

For backward compatibility, the default is "@/etc/nix/machines", or
"@<filename>" for each file name in NIX_REMOTE_SYSTEMS.

src/libstore/globals.hh

@@ -138,13 +138,9 @@ public:
     PathSetting buildHook{this, true, nixLibexecDir + "/nix/build-remote", "build-hook",
         "The path of the helper program that executes builds to remote machines."};
 
-    Setting<std::string> builders{this, "", "builders",
+    Setting<std::string> builders{this, "@" + nixConfDir + "/machines", "builders",
         "A semicolon-separated list of build machines, in the format of nix.machines."};
 
-    Setting<Strings> builderFiles{this,
-        {nixConfDir + "/machines"}, "builder-files",
-        "A list of files specifying build machines."};
-
     Setting<off_t> reservedSize{this, 8 * 1024 * 1024, "gc-reserved-space",
         "Amount of reserved disk space for the garbage collector."};