gdbarch: Remove displaced_step_free_closure

The displaced_step_free_closure gdbarch hook allows architectures to
free data they might have allocated to complete a displaced step.
However, all architectures using that hook use the
simple_displaced_step_free_closure provided in arch-utils.{c,h}, which
does a simple xfree.  We can remove it and do an xfree directly instead
of calling the hook.

gdb/ChangeLog:

	* gdbarch.sh (displaced_step_free_closure): Remove.
	* gdbarch.h, gdbarch.c: Re-generate.
	* aarch64-linux-tdep.c (aarch64_linux_init_abi): Don't set
	displaced_step_free_closure.
	* amd64-linux-tdep.c (amd64_linux_init_abi_common): Likewise.
	* arm-linux-tdep.c (arm_linux_init_abi): Likewise.
	* i386-linux-tdep.c (i386_linux_init_abi): Likewise.
	* rs6000-aix-tdep.c (rs6000_aix_init_osabi): Likewise.
	* rs6000-tdep.c (rs6000_gdbarch_init): Likewise.
	* s390-linux-tdep.c (s390_gdbarch_init): Likewise.
	* arch-utils.h (simple_displaced_step_free_closure): Remove.
	* arch-utils.c (simple_displaced_step_free_closure): Remove.
	* infrun.c (displaced_step_clear): Call xfree instead of
	gdbarch_displaced_step_free_closure.