Remove InternalValidate patch to plugin-sdk

[VenelinMartinov]

Hello!

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Issue details

After #1854 the patch we added before on the plugin-sdk is no longer necessary: pulumi/terraform-plugin-sdk#37

We should be able to remove it freely, there are also tests covering the behaviour.

Note that due to our system with replace, this inevitable causes some user pain, so we might want to package the change with other changes.

Affected area/feature

[VenelinMartinov]

This is wrong - the patch still offers a performance optimisation for large providers - the only unnecessary thing is the global variable, which doesn't really hurt either.