Remove unused DirectoryModel.showSpecialSearchRoots_.

It looks to be used in "old UI" Files.app but it is now gone.

Showing "recent" or "offline" in save dialogs are indeed meaningless,
but it is at least safe (because we are disabling the save button there),
and it has a usecase to save to 'shared with me'.
Altogether, I think just removing the flag variable and not
doing any special handling is the easiest solution.

BUG=none

Review URL: https://codereview.chromium.org/77843002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236247 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236247