compress/flate: use built-in clear to simplify the code

[apocelipes]

The new bootstrap toolchain allows us to use the built-in clear.

Updates #64751

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Message from Dmitri Shuralyov:

Patch Set 1: Commit-Queue+1

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Message from Dmitri Shuralyov:

Patch Set 1: -Commit-Queue

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Message from Ian Lance Taylor:

Patch Set 1: Auto-Submit+1 Code-Review+2 Commit-Queue+1

(1 comment)

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This PR is being closed because golang.org/cl/610516 has been merged.