[refactor](fe) Remove bilingual descriptions from variable annotations

[lsy3993]

What problem does this PR solve?

Issue Number: N/A

Problem Summary: Session and global variable annotations currently store both Chinese and English descriptions as a two-element array. The Chinese description is unused and the array shape makes callers cumbersome. This change keeps only the English description, updates the VarAttr annotation type, and adjusts the affected unit tests.

Release note

None

Check List (For Author)

• Test: Unit Test
  • ./run-fe-ut.sh --run org.apache.doris.qe.SessionVariablesTest
• Behavior changed: No
• Does this need documentation: No

[hello-stephen]

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