* Simplify the substitute mechanism:

- Drop the store expression.  So now a substitute is just a
    command-line invocation (a program name + arguments).  If you
    register a substitute you are responsible for registering the
    expression that built it (if any) as a root of the garbage
    collector.
  - Drop the substitutes-rev DB table.

src/libstore/store.hh

@@ -14,10 +14,6 @@ using namespace std;
    network). */
 struct Substitute
 {
-    /* Store expression to be normalised and realised in order to
-       obtain `program'. */
-    Path storeExpr;
-
     /* Program to be executed to create the store path.  Must be in
        the output path of `storeExpr'. */
     Path program;
@@ -73,6 +69,9 @@ void registerSubstitutes(const Transaction & txn,
 /* Return the substitutes expression for the given path. */
 Substitutes querySubstitutes(const Path & srcPath);
 
+/* Deregister all substitutes. */
+void clearSubstitutes();
+
 /* Register the validity of a path. */
 void registerValidPath(const Transaction & txn, const Path & path);