target/aarch64: fix minor stepping issue with gdb

target/aarch64: fix minor stepping issue with gdb

when using step command from gdb the step happens without any issue,
but aarch64_step call explicitly aarch64_poll which consumes the
status change to HALTED, so it does not inform gdb that the step has
finished.

by removing this call, all is back to normal and openocd could inform gdb
that the step has finished.

Change-Id: I9366aecd20f7d52259b050b8653189b67d9299d0
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5354
Tested-by: jenkins
Reviewed-by: Muhammad Omair Javaid <omair.javaid@linaro.org>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>