arm-tdep.c: Remove unused arm_displaced_step_copy_insn

arm-tdep.c: Remove unused arm_displaced_step_copy_insn

This function is never used, since it is superseded by
arm_linux_displaced_step_copy_insn.

gdb/ChangeLog:

	* arm-tdep.c (arm_displaced_step_copy_insn): Remove.
	(ARM displaced stepping support): Remove reference to
	arm_displaced_step_copy_insn in comment.
	* arm-tdep.h (arm_displaced_step_copy_insn): Remove.
	* arm-linux-tdep.c (arm_linux_displaced_step_copy_insn): Remove
	reference to arm_displaced_step_copy_insn in comment.