Cleanup Codes related to FileDB

[Okabe-Junya]

What this PR does:

Remove all FileDB-related code from the codebase

Why we need it:

See #5652

Which issue(s) this PR fixes:

Ref. #5652
(Not close, since I haven't cleanup-ed docs yet / I haven't decided how to migrate yet)

Note:

Per PipeCD’s maintenance and release policy, you’re welcome to target this PR at a different branch or leave it unmerged for a while.
I respect the maintenance team’s policy :)

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change: Users will no longer be able to configure FileDB as PipeCD datastore.
• How to migrate (if breaking change):

[t-kikuc]

LGTM

[khanhtc1202]

Hi @Okabe-Junya Thanks for the commit 👍
This feature deprecation issue could be a bit more complicated than the removed code in this PR, so we added hold label for now. If you feel interest on this issue, please join the public dev mtg or CNCF #pipecd channel for further discussion.
We really appreciate your interest, thank you 😁

[Okabe-Junya]

Thank you - I understand how complex this issue can become, and I will fully respect the team’s opinions.

Thanks also for sharing the Slack channel and meeting details; I’ll check them out :)