Do not delete flash bag messages on get.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | --
| Fixed tickets | --
| License       | MIT
| Doc PR        | --

I.e. LogUtil::getErrorMessages() relys on flashbag messages not being deleted on get. Otherwise this is a BC-break.