Remove some unused variables

Remove some unused variables

Found using the newly-enabled -Wunused-variable.

gdb/ChangeLog:

	* aarch64-linux-nat.c
	(aarch64_linux_nat_target::stopped_data_address): Remove unused
	variable.
	* arm-linux-nat.c (fetch_regs): Likewise.
	(store_regs): Likewise.
	(fetch_vfp_regs): Likewise.
	(store_vfp_regs): Likewise.
	(arm_linux_nat_target::insert_hw_breakpoint): Likewise.
	(arm_linux_nat_target::remove_hw_breakpoint): Likewise.
	(arm_linux_nat_target::insert_watchpoint): Likewise.
	(arm_linux_nat_target::remove_watchpoint): Likewise.
	* mips-linux-nat.c (mips_linux_nat_target::insert_watchpoint):
	Likewise.
	* nat/aarch64-sve-linux-ptrace.c (aarch64_sve_get_sveregs):
	Likewise.
	* ppc-linux-nat.c (fetch_register): Likewise.
	(fetch_all_gp_regs): Likewise.
	(fetch_ppc_registers): Likewise.
	(store_all_gp_regs): Likewise.
	(store_ppc_registers): Likewise.
	(hwdebug_insert_point): Likewise.
	(can_use_watchpoint_cond_accel): Likewise.
	* remote-sim.c (gdb_os_write_stdout): Likewise.

gdb/gdbserver/ChangeLog:

	* linux-mips-low.c (mips_collect_ptrace_register): Remove unused
	variable.
	(mips_supply_ptrace_register): Likewise.