fix(manager): removes outdated syncTarget field

[danwt]

PR Standards

Opening a pull request should be able to meet the following requirements

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Close https://github.com/dymensionxyz/dymint/issues/850

<-- Briefly describe the content of this pull request -->

For Author:

• [ ] Targeted PR against correct branch
• [ ] included the correct type prefix in the PR title
• [ ] Linked to Github issue with discussion and accepted design
• [ ] Targets only one github issue
• [ ] Wrote unit and integration tests
• [ ] All CI checks have passed
• [ ] Added relevant godoc comments

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For Reviewer:

• [ ] confirmed the correct type prefix in the PR title
• [ ] Reviewers assigned
• [ ] confirmed all author checklist items have been addressed

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After reviewer approval:

• [ ] In case targets main branch, PR should be squashed and merged.
• [ ] In case PR targets a release branch, PR should be rebased.

[omritoptix]

@danwt conflicts

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.48780% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 39.14%. Comparing base (851b312) to head (448db03). Report is 33 commits behind head on main.

:exclamation: Current head 448db03 differs from pull request most recent head 74ce466. Consider uploading reports for the commit 74ce466 to get more accurate results

Files	Patch %	Lines
block/synctarget.go	11.11%	8 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #854 +/- ## ========================================== - Coverage 40.44% 39.14% -1.31% ========================================== Files 120 120 Lines 18935 18602 -333 ========================================== - Hits 7659 7282 -377 - Misses 10201 10281 +80 + Partials 1075 1039 -36 ```

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