Eliminate procfs.c:procfs_use_watchpoints

Now that procfs.c is only ever used by Solaris, and, both x86 and
SPARC Solaris support watchpoints (*), we don't need the separate
procfs_use_watchpoints function.  Getting rid of it simplifies
C++ification of target_ops.

(*) and I assume that any other Solaris port would use the same kernel
debug API interfaces for watchpoints.  Otherwise, we can worry about
it if it ever happens.

gdb/ChangeLog:
2018-05-02  Pedro Alves  <palves@redhat.com>

	* procfs.c (procfs_stopped_by_watchpoint)
	(procfs_insert_watchpoint, procfs_remove_watchpoint)
	(procfs_region_ok_for_hw_watchpoint, procfs_stopped_data_address):
	Forward declare.
	(procfs_use_watchpoints): Delete, move contents...
	(procfs_target): ... here.
	* procfs.h (procfs_use_watchpoints): Delete declaration.
	* i386-sol2-nat.c (_initialize_amd64_sol2_nat): Don't call
	procfs_use_watchpoints.
	* sparc-sol2-nat.c (_initialize_sparc_sol2_nat): Don't call
	procfs_use_watchpoints.