Eliminate target_have_continuable_watchpoint

Eliminate target_have_continuable_watchpoint

target_have_continuable_watchpoint isn't used anywhere so remove it.
The property isn't necessary because checking for "continuable" is the
same as checking for "!steppable && !non-steppable".

gdb/ChangeLog:
2018-08-31  Pedro Alves  <palves@redhat.com>

	* nto-procfs.c (nto_procfs_target::have_continuable_watchpoint):
	Delete.
	* s390-linux-nat.c
	(s390_linux_nat_target::have_continuable_watchpoint): Delete.
	* target.h (target_ops::have_continuable_watchpoint): Delete.
	(target_have_continuable_watchpoint): Delete.
	* x86-nat.h (x86_nat_target::have_continuable_watchpoint): Delete.
	* target-delegates.c: Regenerate.