minor #15066 [FrameworkBundle] Remove unused old_assets.xml (nicolas-grekas)

This PR was merged into the 2.7 branch.

Discussion
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[FrameworkBundle] Remove unused old_assets.xml

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commit f74a1f2dc34d8b141e1f4bfbbfa1dbb7d955fe42 removed any reference to old_assets.xml

Commits
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18e37c8 [FrameworkBundle] Remove unused old_assets.xml

Author: Tobion