don't specify a chromedriver version in example yml
don't specify a chromedriver version in example yml

after g/255045, we don't need to override the version jenkins uses
to do local selenium testing anymore, so simply removing the line
saves the trouble of updating it every time there's a new version
of chrome

test plan:
 - remove chromedriver_version from config/selenium.yml
 - you should be able to run selenium specs locally

Change-Id: Ia2ccd9fbe6ed518adbcc9c6cf0755e4e22f4507a
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/279761
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: James Butters <4c5a489c9bb72d41f20b3b22aaf4c9b864e32f20@instructure.com>
QA-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
Product-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms