[bugfix] Remove errant alsoKnownAs inline @context entry (#4280)

# Description

> If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.
>
> If this is a documentation change, please briefly describe what you've changed and why.

Our extra `alsoKnownAs` inline `@context` entry was incorrect, and doesn't need to be included anyways as it's already present at the AS namespace context doc here: https://www.w3.org/ns/activitystreams. This PR just removes it :)

closes https://codeberg.org/superseriousbusiness/gotosocial/issues/3883

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [ ] I/we have commented the added code, particularly in hard-to-understand areas.
- [ ] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4280
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

vendor/modules.txt

@@ -1,4 +1,4 @@
-# code.superseriousbusiness.org/activity v1.15.0
+# code.superseriousbusiness.org/activity v1.15.1
 ## explicit; go 1.23
 code.superseriousbusiness.org/activity/pub
 code.superseriousbusiness.org/activity/streams