Revert "gdb: support zero inode in generate-core-file command"

This reverts commit 1e21c846c275fc6e387ca903a129096be2a53d0b.

This change was causing unexpected mappings to be included in the core
files generated by GDB, which was triggering warnings when GDB opened
a core file, like this:

  warning: Can't open file [stack] during file-backed mapping note processing

  warning: Can't open file [vvar] during file-backed mapping note processing

For now I'm reverting the above commit and will come to the list again
when I have a solution that addresses the original issue without also
including the unexpected mappings.