[DiffTrain] Strip @license from files (#25821)

We need to remove this for some internal tests. This was previously in
our upgrade script so adding it back here for parity.

Test plan: files were updated correctly
[[1]](https://github.com/facebook/react/commit/1704bbb8268345bf7aa405101313b3639e03e221#diff-80b968e05ce2ceeff6e17c938dc722aff2c1c660f8ef402e6664bc5b2cafa5fbL2)
[[2]](https://github.com/facebook/react/commit/1704bbb8268345bf7aa405101313b3639e03e221#diff-6ecd07a61c8e0e28793dace3f9f62751e4808de9c2965aa1ddeb0157f3c9b4ecL2)

Co-authored-by: Jan Kassens <jkassens@meta.com>