[close #409] build: Remove "prepare_test_binaries" flag

[pingyu]

What problem does this PR solve?

Issue Number: close #409

Problem Description: build: Cannot re-download 3rd binaries after "make clean"

What is changed and how does it work?

• Remove "prepare_test_binaries" flag in make clean.

Code changes

• No code

Check List for Tests

This PR has been tested by at least one of the following methods:

• Integration test

Side effects

• No side effects

Related changes

• No related changes

To reviewers

Please follow these principles to check this pull requests:

• Concentration. One pull request should only do one thing. No matter how small it is, the change does exactly one thing and gets it right. Don't mix other changes into it.
• Tests. A pull request should be test covered, whether the tests are unit tests, integration tests, or end-to-end tests. Tests should be sufficient, correct and don't slow down the CI pipeline largely.
• Functionality. The pull request should implement what the author intends to do and fit well in the existing code base, resolve a real problem for TiDB/TiKV users. To get the author's intention and the pull request design, you could follow the discussions in the corresponding GitHub issue or internal.tidb.io topic.
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(The above text mainly refers to TiDB Development Guide. It's also highly recommended to read about Writing code review comments)

[zeminzhou]

LGTM