wasm: remove free symbol export

[Shikugawa]

What this PR does / why we need it:
export _free is required any more with this fix envoyproxy/envoy-wasm#404, No?

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[istio-testing]

Hi @Shikugawa. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[mandarjog]

/ok-to-test