Remove explicit thread destruction code in the libasync driver.
Remove explicit thread destruction code in the libasync driver.

See the discussion in #1835. Libasync now uses the default thread pool of std.parallelism, which is shut down automatically.

Geod24/vibe.d,rejectedsoftware/vibe.d,etcimon/vibe.d,DBankov/vibe.d,chalucha/vibe.d,gedaiu/vibe.d,etcimon/vibe.d