More procfs.c simplification

There are only two architectures using procfs.c (i386/SPARC), and none
of their corresponding nat files overrides any target method.  Move
the add_target calls to procfs.c directly.

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

	* i386-sol2-nat.c (_initialize_amd64_sol2_nat): Don't call
	procfs_target/add_target here.
	* procfs.c (procfs_target): Make static.
	(_initialize_procfs): Call add_target here.
	* procfs.h (struct target_ops): Remove forward declaration.
	(procfs_target): Remove declaration.
	* sparc-sol2-nat.c (_initialize_sparc_sol2_nat): Delete.