Revert "Stdio.Buffer: Explicitly clear io->this to mark destruction."

This reverts commit 73003705.

This commit was part of a set of speculative fixes that
introduced other issues (SIGSEGV in testsuite).

src/modules/_Stdio/buffer.cmod

@@ -2854,7 +2854,6 @@ PIKECLASS Buffer
 
   EXIT {
     Buffer *io = THIS;
-    io->this = 0;
     io_unlink_external_storage( io );
     if( io->error_mode )
         free_program( io->error_mode );