[WebProfilerBundle] Simplified session storage implementation
minor #10021 [WebProfilerBundle] Simplified session storage implementation (bschussek)

This PR was merged into the 2.5-dev branch.

Discussion
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[WebProfilerBundle] Simplified session storage implementation

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

Follow-up to #9967.

Commits
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cec05bf [WebProfilerBundle] Simplified session storage implementation