Remove redundant and racy assertion

The `wait_tagged_idle_count/1` function already performs an assertion
[1] on the idle count, so the final `?assertEqual` is both redundant,
and also apparently racy based on this test failure:

```
::in function couchdb_os_proc_pool:if_no_tagged_process_found_new_must_be_spawned/1 (test/eunit/couchdb_os_proc_pool.erl, line 397)
in call from eunit_test:run_testfun/1 (eunit_test.erl, line 71)
in call from eunit_proc:run_test/1 (eunit_proc.erl, line 543)
in call from eunit_proc:with_timeout/3 (eunit_proc.erl, line 368)
in call from eunit_proc:handle_test/2 (eunit_proc.erl, line 526)
in call from eunit_proc:tests_inorder/3 (eunit_proc.erl, line 468)
in call from eunit_proc:with_timeout/3 (eunit_proc.erl, line 358)
in call from eunit_proc:run_group/2 (eunit_proc.erl, line 582)
**error:{assertEqual,[{module,couchdb_os_proc_pool},
              {line,397},
              {expression,"idle_count ( )"},
              {expected,3},
              {value,2}]}
```
It seems that the tagged idle count can also drop from 3 to 2 between
the first and second assertion.

[1] https://github.com/apache/couchdb/blob/main/src/couch/test/eunit/couchdb_os_proc_pool.erl#L654

Author: jaydoane

src/couch/test/eunit/couchdb_os_proc_pool.erl

@@ -393,8 +393,7 @@ if_no_tagged_process_found_new_must_be_spawned(_) ->
     % After 3rd proc returns to the pool there should
     % be 3 tagged idle processes
     ?assertEqual(ok, stop_client(Client3)),
-    wait_tagged_idle_count(3),
-    ?assertEqual(3, idle_count()).
+    wait_tagged_idle_count(3).
 
 db_tag_none_works(_) ->
     cfg_set("db_tag", "none"),