Remove aion_api submodule

[aion-kelvin]

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Description

Please include a brief summary of the change that this pull request proposes. Include any relevant motivation and context. List any dependencies required for this change.

• Remove aion_api as a submodule from aion
• ./gradlew pack will no longer have the aion api (it will be distributed separately in the future)
• Related: Gradle: separate from aion repo and build scripts aion_api#60

Fixes Issue # .

Type of change

Insert x into the following checkboxes to confirm (eg. [x]):

• Bug fix.
• New feature.
• Enhancement.
• Unit test.
• Breaking change (a fix or feature that causes existing functionality to not work as expected).
• Requires documentation update.

Testing

Please describe the tests you used to validate this pull request. Provide any relevant details for test configurations as well as any instructions to reproduce these results.

• compiled and did pack build, then made sure the new pack output's kernel can start up

Verification

Insert x into the following checkboxes to confirm (eg. [x]):

• I have self-reviewed my own code and conformed to the style guidelines of this project.
• New and existing tests pass locally with my changes.
• I have added tests for my fix or feature.
• I have made appropriate changes to the corresponding documentation.
• My code generates no new warnings.
• Any dependent changes have been made.

[AionJayT]

LGTM