Revert "Add the pre-build hook."

Going to reimplement differently.

This reverts commit 1e4a4a2e9f.

src/libstore/globals.hh

@@ -206,10 +206,6 @@ struct Settings {
     /* Whether the importNative primop should be enabled */
     bool enableImportNative;
 
-    /* The hook to run just before a build to set derivation-specific
-       build settings */
-    Path preBuildHook;
-
 private:
     SettingsMap settings, overrides;