nexus: update example configuration (#8787)

Updated Nexus' example configuration to no longer require regions to
allocate on distinct sleds. This allows a local simulated Omicron
started by `cargo xtask omicron-dev run-all` to successfully create disk
resources rather than error with insufficient capacity, allowing us to
run acceptance tests for the Go SDK and Terraform provider against this
environment.

A few OpenAPI generation scripts have also been removed that were deemed
unnecessary. OpenAPI generation is handled by `cargo xtask openapi
generate` these days.

Author: sudomateo

nexus/examples/config.toml

@@ -140,10 +140,10 @@ sp_ereport_ingester.period_secs = 30
 
 [default_region_allocation_strategy]
 # allocate region on 3 random distinct zpools, on 3 random distinct sleds.
-type = "random_with_distinct_sleds"
+# type = "random_with_distinct_sleds"
 
 # the same as random_with_distinct_sleds, but without requiring distinct sleds
-# type = "random"
+type = "random"
 
 # setting `seed` to a fixed value will make dataset selection ordering use the
 # same shuffling order for every region allocation.