Remove duplicated mish activation function
# What does this PR do?
The mish activation function was repeated in the file. I removed the duplicate.

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dimiss the other checks if that's the case).

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors which may be interested in your PR.

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