[maps] fix Map orphans sources on layer deletion - #159067

Fixes elastic#158133

Sources where not getting removed because `return` was used instead of
`continue` in `for...in` loop. This caused the function to return
instead of processing remaining sources.

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
(cherry picked from commit fcd1950)

…9160)

# Backport

This will backport the following commits from `main` to `8.8`:
- [[maps] fix Map orphans sources on layer deletion
(#159067)](#159067)

<!--- Backport version: 8.9.7 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

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`for...in` loop. This caused the function to return\r\ninstead of
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fix Map orphans sources on layer deletion (#159067)\n\nFixes
https://github.com/elastic/kibana/issues/158133\r\n\r\nSources where not
getting removed because `return` was used instead of\r\n`continue` in
`for...in` loop. This caused the function to return\r\ninstead of
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fix Map orphans sources on layer deletion (#159067)\n\nFixes
https://github.com/elastic/kibana/issues/158133\r\n\r\nSources where not
getting removed because `return` was used instead of\r\n`continue` in
`for...in` loop. This caused the function to return\r\ninstead of
processing remaining sources.\r\n\r\n---------\r\n\r\nCo-authored-by:
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Co-authored-by: Nathan Reese <reese.nathan@elastic.co>