build-system: Don't auto-clean build dir on repeated failure.

This was needed prior to CMake (and for some time as we were porting to
CMake) so that a wedged build system had at least a hope of eventually
being corrected.

This interferes with other use cases however and since CMake is really
good nowadays at generating valid build systems, remove this behavior.
Users can still pass --refresh-build when desired.

Fixes #30.
2 jobs for master in 1 minute and 51 seconds (queued for 3 minutes and 15 seconds)
Status Job ID Name Coverage
  Test
passed #9425
smoke-test

00:01:17

passed #9424
unit-test

00:00:33