Replace delete_longjmp_breakpoint_cleanup with a forward_scope_exit type

Replace delete_longjmp_breakpoint_cleanup with a forward_scope_exit type

This removes delete_longjmp_breakpoint_cleanup in favor of forward_scope_exit.

gdb/ChangeLog:
2019-01-23  Tom Tromey  <tom@tromey.com>
	    Andrew Burgess  <andrew.burgess@embecosm.com>
	    Pedro Alves  <palves@redhat.com>

	* breakpoint.c (until_break_command): Use
	delete_longjmp_breakpoint_cleanup class.
	* infcmd.c (delete_longjmp_breakpoint_cleanup): Remove function.
	(until_next_command): Use delete_longjmp_breakpoint_cleanup class.
	* inferior.h: Include forward-scope-exit.h.
	(delete_longjmp_breakpoint_cleanup): Replace function declaration
	with FORWARD_SCOPE_EXIT type.