Form to subscribe to gdb mailing lists

[Simon Marchi]

It has been reported [1] that the form on the following page to subscribe to mailing lists is broken,
probably due to the recent infrastructure changes:

https://www.gnu.org/software/gdb/mailing-lists/

I propose that:

1. We remove this form
2. We change the mailing list links to point to the "listinfo" links, where people can register

For example, that means changing the "gdb" link from

  https://sourceware.org/pipermail/gdb/

to

  https://sourceware.org/mailman/listinfo/gdb

The first link leads to the archives, but the listinfo page has a link to the archives, so it's not lost.

Simon

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=25906