[Files.app] Remove the ensureItemNotBehindPreviewPanel hack from the file list

r238291 fixes the padding bug on list.js, and now the selected item is no longer behind the preview panel. Hence we can remove this hack from the code.

BUG=324819
TEST=manually tested

Review URL: https://codereview.chromium.org/103733002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238683 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238683