[dexter] Don't generate results files by default

Dexter saves various files to a new results directory each time it is run
(including when it's run by lit tests) and there isn't a way to opt-out. This
patch reconfigures the behaviour to be opt-in by removing the default
`--results-directory` location. Now results are only saved if
`--results-directory` is specified.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D119545