sdkserver: add functionality to remove game server annotations

[antiphp]

Is your feature request related to a problem? Please describe.
When the game server resets itself, e.g. from being allocated back to ready, it is required to reset exposed information like annotations too.

Describe the solution you'd like
I suggest to implement a function for the sdk server to remove an annotation. The recommended way to expose information from the game server is among others setting annotations (https://agones.dev/site/docs/faq/#how-can-i-expose-information-from-my-game-server-binary-to-an-external-service). This should include removing a previously set annotation.

Describe alternatives you've considered
Overriding with empty string.

Additional context
Related code: https://github.com/googleforgames/agones/blob/v1.43.0/pkg/sdkserver/sdkserver.go#L582

[antiphp]

Accepting an empty string to delete the annotation would be BC, so this must be separate function.

Would open a PR if or when #3835 has been resolved.

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