Revert "webui: Avoid useless backend header in single-backend mode"

This reverts commit 862b6ae7a77c2447383b8a90199445caeccdc1b9.
webui: Avoid useless backend header in single-backend mode

This time, do not break the multibackend with only one backend case.
Merge pull request #206 from vuntz/ui-fix-header

Fix the fix for useless backend header in UI