drop support for CONCURRENT_REQUESTS_PER_SPIDER
Drop old engine code

  * remove Downloader import unused since 1fba64
  * remove CONCURRENT_SPIDERS deprecation warning from a1dbc6 (2011)
Merge pull request #911 from nyov/nyov/dev

Drop old engine code
Merge pull request #895 from scrapy/we-are-past-0.15

[MRG] drop support for CONCURRENT_REQUESTS_PER_SPIDER