examples: drop specific-commit deps for dirs/term

[daviddrysdale]

Commit d362ebd ("Update Rust version (#588)") upgraded the
'dirs' (2.0.1->2.0.2) and 'term' (0.5.2->0.6.1) dependencies so
the pins to particular commits are no longer needed.

[tiziano88]

Just checking: will rustfmt still build after removing these? I don't remember whether they were added for it, and now that that example is disabled, I wouldn't want us to accidentally remove some deps that are needed.

[daviddrysdale]

The rustfmt example built for me (once I remembered to update my local Rust compiler), but I guess Cloudbuild will tell us definitively. (AIUI the rustfmt example doesn't get run at the moment (#594) but should get built by the run_tests step).