Remove img_modify_playbook assignation in build_yardstick_image.yml

Remove img_modify_playbook assignation in build_yardstick_image.yml

Variable "img_modify_playbook" is being assigned in first context of
./ansible/build_yardstick_image.yml [1]. However this variable is never used.
In second context [2], the variable used is a CLI input variable.

[1]https://github.com/opnfv/yardstick/blob/stable/euphrates/ansible/build_yardstick_image.yml#L187
[2]https://github.com/opnfv/yardstick/blob/stable/euphrates/ansible/build_yardstick_image.yml#L205

JIRA: YARDSTICK-871

Change-Id: I7bd5063c5e86520abcafdc782222ee53faf58f45
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>