Delete exit_inferior_silent(int pid)

In commit 00431a78 ("Use thread_info and inferior pointers more
throughout"), I introduced 'exit_inferior_silent(inferior *)' and left
'exit_inferior_silent(int)' behind by mistake.  This overload isn't
used anywhere.  It's not declared in inferior.h either, that commit
deleted its declaration in favor of the new
'exit_inferior_silent(inferior *)'.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* inferior.c (exit_inferior_silent(int)): Delete.