wasm: delete unused code.

[mathetake]

Wasm is already upstreamed to Envoy and this PR removes the unused codes from here. Notably this removes IstioContext, IstioWasm and thus the factory overrides over EnvoyWasm class.
Ref: envoyproxy/envoy#17268

Signed-off-by: Takeshi Yoneda takeshi@tetrate.io

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[istio-testing]

Hi @mathetake. 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.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[mathetake]

cc @PiotrSikora

[bianpengyuan]

/ok-to-test

[bianpengyuan]

/retest