[fix] bar graph and rename CSS class engine-scores -> engine-score

- drop #main_stats selector in stats.less
- 'engine-score' exists before this PR.
- untabify searx/static/themes/__common__/less/stats.less

for details see comment at: https://github.com/searxng/searxng/pull/81/files/d93bec7638908cae64530727d4e1539ae10bd144..1204e4f07e932029d66b4674a3474918228459a6#r633571496



Suggested-by: @dalf in commit 1204e4f0
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>