[fix] simple theme - remove no longer used stats.less file

File searx/static/themes/simple/less/stats.less is not used (imported) in any
other less file.  I can't say when it's usage was dropped or if it has ever been
used.  ATM this file is without any usage.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>