remove dead make testgen target
remove dead make testgen target

Summary:
the testgen folder was deleted in 2019: D17739878 (https://github.com/facebook/flow/commit/6383cd53f5be2cab30e42dc7fc5e4f9b53e8a33e)

Changelog: [internal]

Reviewed By: SamChou19815

Differential Revision: D37591064

fbshipit-source-id: c4f19bb90301533b9674b5aa71524afb86b63fce

mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow