Darwin: Remove unused variables [-Werror=unused-variable]

Building GDB with --host=x86_64-apple-darwin15 using G++ 5.3.0 runs
into a number of -Werror=unused-variable warnings/errors:

  ../../src/gdb/i386-darwin-nat.c: In function 'void i386_darwin_dr_set(int, CORE_ADDR)':
  ../../src/gdb/i386-darwin-nat.c:283:7: error: unused variable 'current_pid' [-Werror=unused-variable]
     int current_pid;
	 ^
  [etc, etc.]

This commit fixes all such warnings.

gdb/ChangeLog:
2018-09-30  Pedro Alves  <palves@redhat.com>

	* darwin-nat-info.c (darwin_debug_regions_recurse)
	(info_mach_exceptions_command): Remove unused local variables.
	* darwin-nat.c (darwin_decode_notify_message)
	(darwin_nat_target::resume, darwin_nat_target::mourn_inferior)
	(darwin_stop_inferior, darwin_setup_exceptions)
	(darwin_nat_target::kill, darwin_attach_pid, darwin_ptrace_him)
	(darwin_nat_target::attach, darwin_nat_target::detach)
	(darwin_read_write_inferior, darwin_read_dyld_info): Remove unused
	local variables.
	* i386-darwin-nat.c (i386_darwin_dr_set): Remove unused local
	variables.