gdb/continuations: remove the 'err' from 'do_all_inferior_continuations'

gdb/continuations: remove the 'err' from 'do_all_inferior_continuations'

The 'err' parameter of 'do_all_inferior_continuations' is effectively
unused.  There is only one place where the function is called, and
there the argument is a literal 0.  So, remove the parameter.

gdb/ChangeLog:
2021-04-22  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* continuations.h (do_all_inferior_continuations): Remove the 'err'
	parameter.  Update the references below.
	* continuations.c (do_my_continuations_1)
	(do_my_continuations)
	(do_all_inferior_continuations): Update.
	* inf-loop.c (inferior_event_handler): Update.
	* infcmd.c (attach_command_continuation): Update.