Remove unused struct fields

Hi hackers,

Here are a few fields that are not used.

1) username in scram_state:

Added and used in 818fd4a67d61.
Usage removed in f3049a603a7.

2) mask in PgAioWorkerSubmissionQueue:

Added in and not used since 247ce06b883d.

3) write_page in SharedTuplestoreAccessor:

Added in and not used since ab9e0e718acb.

Those have been found with a coccinelle script [1].

Note that it generates a lot of false positives and requires careful manual
review. I'll try to improve it if time permits.

[1]: https://github.com/bdrouvot/coccinelle_on_pg/blob/main/misc/unused_struct_fields.cocci

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com