Ensure that Perl processes delete their entry in the temproots directory

By moving the destructor object to libstore.so, it's also run when
download-using-manifests and nix-prefetch-url exit.  This prevents
them from cluttering /nix/var/nix/temproots with stale files.

src/libmain/shared.hh

@@ -48,11 +48,6 @@ extern bool setuidMode;
 
 extern volatile ::sig_atomic_t blockInt;
 
-struct RemoveTempRoots 
-{
-    ~RemoveTempRoots();    
-};
-
 /* Exit code of the program. */
 extern int exitCode;