gdb: make inferior::clear_thread_list always silent

After this commit:

  commit a78ef875
  Date:   Wed Jun 22 18:10:00 2022 +0100

      Always emit =thread-exited notifications, even if silent

The function mi_interp::on_thread_exited (or mi_thread_exit as the
function was called back then) no longer makes use of the "silent"
parameter.

As a result there is no difference between inferior::clear_thread_list
with silent true or false, because:

  - None of the interpreter ::on_thread_exited functions rely on the
    silent parameter, and

  - None of GDB's thread_exit observers rely on the silent parameter
  either.

This commit removes the silent parameter from
inferior::clear_thread_list, and makes the function always silent.

This commit was originally part of a larger series:

  https://inbox.sourceware.org/gdb-patches/20221212203101.1034916-1-pedro@palves.net/

But didn't really need to be part of that series.  I had an interest
in seeing this patch merged:

  https://inbox.sourceware.org/gdb-patches/20221212203101.1034916-31-pedro@palves.net/



Which also didn't really need to be part of the larger series, but
does depend, at least a little, on this commit.  In order to get the
fix I'm interested in merged quicker, I (Andrew Burgess) have rebased
this commit outside of the original series.  Any bugs introduced while
splitting this patch out and rebasing, are entirely my own.

There should be no user visible changes after this commit.

Co-Authored-By: Andrew Burgess <aburgess@redhat.com>