Remove setting of the immutable bit

Using the immutable bit is problematic, especially in conjunction with
store optimisation.  For instance, if the garbage collector deletes a
file, it has to clear its immutable bit, but if the file has
additional hard links, we can't set the bit afterwards because we
don't know the remaining paths.

So now that we support having the entire Nix store as a read-only
mount, we may as well drop the immutable bit.  Unfortunately, we have
to keep the code to clear the immutable bit for backwards
compatibility.

src/libutil/immutable.cc

@@ -16,7 +16,7 @@
 namespace nix {
 
 
-void changeMutable(const Path & path, bool mut)
+void makeMutable(const Path & path)
 {
 #if defined(FS_IOC_SETFLAGS) && defined(FS_IOC_GETFLAGS) && defined(FS_IMMUTABLE_FL)
 
@@ -38,30 +38,12 @@ void changeMutable(const Path & path, bool mut)
     /* Silently ignore errors getting/setting the immutable flag so
        that we work correctly on filesystems that don't support it. */
     if (ioctl(fd, FS_IOC_GETFLAGS, &flags)) return;
-
     old = flags;
-    
-    if (mut) flags &= ~FS_IMMUTABLE_FL;
-    else flags |= FS_IMMUTABLE_FL;
-
+    flags &= ~FS_IMMUTABLE_FL;
     if (old == flags) return;
-
     if (ioctl(fd, FS_IOC_SETFLAGS, &flags)) return;
-    
 #endif
 }
 
 
-void makeImmutable(const Path & path)
-{
-    changeMutable(path, false);
-}
-
-
-void makeMutable(const Path & path)
-{
-    changeMutable(path, true);
-}
-
-
 }