src: don't use guava's Files#asCharSink

Guava's implementation of Files#asCharSink s marked @Beta and can be replaced
with java native Files#writeString.

Result:
less dependency on guava's non stable API.

Acked-by: Paul Millar
Acked-by: Lea Morschel
Target: master
1 job for master in 1 minute and 20 seconds (queued for 26 seconds)
Status Name Job ID Coverage
  Build
passed build #24589

00:01:20