Remove assert in procfs.c::procfs_make_note_section (x86-solaris)

On x86-solaris, the gcore command sometimes triggers the following
internal error:

    (gdb) gcore
    /[...]/procfs.c:5523: internal-error: procfs_make_note_section: Assertion `thread_args.note_data != note_data' failed.

The problem is extremely elusive, for reasons that will become clearer
as I explain what is going on.

The program used to produce this issue was really simple:

    | void break_me (void) { }
    |
    | int
    | main (void)
    | {
    |   break_me ();
    |   return 0;
    | }

The procfs_make_note_section builds a buffer incrementally with
the contents of the core's notes section.  The interesting bits are:

  char *note_data = NULL;
  [...]
  note_data = (char *) elfcore_write_prpsinfo (obfd,
                                               note_data,
                                               note_size,
                                               fname,
                                               psargs);

This is the first call to bfd's elfcore which initializes note_data.
After that, we have a few more calls, which keep updating notes_data
and note_size, but our interest lies in the following part of
the function:

  thread_args.note_data = note_data;
  [...]
  proc_iterate_over_threads (pi, procfs_corefile_thread_callback,
                             &thread_args);
  /* There should be always at least one thread.  */
  gdb_assert (thread_args.note_data != note_data);

The comment implies that the assert is to verify that our loop
iterated over at least one thread. The check is relying on the
fact that the notes_data returned by the elfcore module changes
at each iteration, via (in procfs_corefile_thread_callback):

      args->note_data = procfs_do_thread_registers (args->obfd, ptid,
                                                    args->note_data,
                                                    args->note_size,
                                                    args->stop_signal);

(which calls elfcore_write_lwpstatus).

But, while it happens most of the time, thanks to a call to realloc
in elfcore_write_note (the function that actually appends the data
at the end of the notes buffer),...

       buf = (char *) realloc (buf, *bufsiz + newspace);

... this is by no means guarantied. In fact, under the right
circumstances, the buffer was grown twice without changing
addresses. Unfortunately, the circumstances are very sensitive,
thus making this bug very elusive.

This patch fixes the problem by simply removing the assert.
This means we're losing the assertion that there is at least one
thread, but I think that's OK. If we still want to keep the
assertion, we have the option of either checking the buffer
size, or else adding a boolean flag in the context structure
that we'd set to true as soon as we have a thread.

gdb/ChangeLog:

        * procfs.c (procfs_make_note_section): Remove assertion and
        associated comment.