kafka/server: Don't provide a topic property override for cleanup_policy_flags

[graphcareful]

Cover letter

• Manually setting an override for this topic attribute displays it as a DYNAMIC_TOPIC_CONFIG in describe topics requests.

• Overriding this is not necessary anyway since the default is always to be taken.

• Contrary to the comment, kafka does not provide an override to the topic attribute, this can be observed by running describe topics command against a fresh topic in kafka. The DEFAULT_CONFIG describe_configs_source will be displayed instead of DYNAMIC_TOPIC_CONFIG.

• This change is introduced to behave closer to as kafka would with the observed topic properties on newly created topics.

Perceived effects with these changes:

➜  redpanda: rpk topic create foo                          
TOPIC  STATUS                                                                                                           
foo    OK                                                                                                               

➜  redpanda: rpk topic describe foo                               
SUMMARY                                                                                                                 
=======                                                                                                                 
NAME        foo                                                                                                         
PARTITIONS  1                                                                                                           
REPLICAS    1

CONFIGS
=======
KEY                           VALUE                          SOURCE
cleanup.policy                delete                         DEFAULT_CONFIG
...

Release notes

• none