[Docs] Remove obsolete Workload Policy session-variable references

[feiniaofeiafei]

Summary

• Remove the obsolete username condition and set_session_variable action from the Workload Policy documentation.
• Remove the related session-variable example and FE/BE isolation note.
• Synchronize the changes across dev, 4.x, 3.x, and 2.1 documentation in English, Chinese, and Japanese.
• Correct the current and 4.x Japanese CREATE WORKLOAD POLICY pages, which incorrectly contained CREATE WORKLOAD GROUP content.

Motivation

The existing documentation described Workload Policy behavior that is no longer supported, which could mislead users. This PR aligns the documentation with the currently supported cancel_query action and keeps all maintained versions and languages consistent.

Versions

• dev
• 4.x
• 3.x
• 2.1 or older (not covered by version/language sync gate)

Languages

• Chinese
• English
• Japanese candidate translation needed

Docs Checklist

• Checked by AI
• Test Cases Built — N/A, documentation-only changes
• Updated required version and language counterparts, or explained why not
• If only one language changed, confirmed whether source/translation counterparts need sync — N/A, all counterparts were updated

Validation

• git diff --check HEAD~2..HEAD
• Website build was not run; changes are documentation-only.