Remove a redundant line in a layout test.

execCommand("SelectAll") depend on
the current position of the selection, as the selection
does not extend outside an editing boundary or a shadow
boundary.

But for this test, we don't need sel.setPosition before it.

BUG=

Review URL: https://codereview.chromium.org/241643005

git-svn-id: svn://svn.chromium.org/blink/trunk@171926 bbb929c8-8fbe-4397-9dbb-9b2b20218538