router: remove useless error

[djshow832]

What problem does this PR solve?

Issue Number: close None

Problem Summary:
When I search "[ERROR]", almost all of the errors are "fetch backend list failed, will retry later".

What is changed and how it works:
Do not report the error when the context is cancelled.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

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None