eliminate flux_reactor_set_priority()

[garlick]

Problem: flux_reactor_set_priority() only works on check watchers, and cannot be easily provided if we replace libev with libuv (see #6492)

Perhaps we could come up with an alternative solution here, the one in-tree user.

[chu11]

There is technically the soution that was done before flux_reactor_set_priority() was added. https://github.com/flux-framework/flux-core/pull/6317/files :-)

Worst comes to worst, perhaps some flags could be added to "emulate" priority. i.e. if certain flags/conditions met, set flags to not run certain watchers "one iteration"