remove closure around @async et al

[bjarthur]

closes #18227 and #19468

[andreasnoack]

cc: @amitmurthy

[bjarthur]

ooh. this is a bigger job than i imagined. every use in Base of async, fetch, fetchfrom, spawn, and spawnat must be audited for the removal of the closure.

[amitmurthy]

We just need to remove the call to localize_vars. And remove localize_vars too as no other code uses it. The expressions still needs to be wrapped in a closure which is what is serialized and executed remotely.