Remove unnecessary check in `merged_s2s`

[dachengx]

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What does the code in this PR do / what does it improve?

Sometimes end_merge_at and start_merge_at can be empty arrays. Function _filter_s1_starts has already guaranteed that the plugin will not merge a single peak.

Can you briefly describe how it works?

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• [ ] Update the docstring(s)
• [ ] Update the documentation
• [ ] Tests to check the (new) code is working as desired.
• [ ] Does it solve one of the open issues on github?

Notes on testing

• Until the automated tests pass, please mark the PR as a draft.
• On the XENONnT fork we test with database access, on private forks there is no database access for security considerations.

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[coveralls]

coverage: 93.439% (-0.01%) from 93.451% when pulling f25a0e4c127ecabb7300ec0cbb9b2d86e51f34ec on rm_merged_s2s_check into 40f75b192a5dfbe81a2220e168086eb307cc1a8b on master.