Update buck version when there's an April 4 or newer release & clean up after #9890

[swolchok]

#9890 employed a workaround for not having facebook/buck2-prelude@958af4f . When there's a buck2 release that has a prelude version containing that commit (the two versions are paired), we need to:

• update buck2 and our prelude git submodule
• clean up the code duplication left behind in Reapply "Depend on extension/threadpool, not thread_parallel_interface, in buck (#9511)" #9890

[swolchok]

still no release yet.

[swolchok]

#10742 updated, closing