unordered_map,unordered_set: Simplify excess list size computation

[stotko]

In #414, the excess list size computation has been fixed for very low capacities. However, the use of std::max is not needed anymore due to the fix. Remove this expression to simplify the computation of the size.

While at it, also make the estimate for the expected collisions slightly more conservative by always rounding up the computed value.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.37%. Comparing base (b7a350e) to head (202c516).

:exclamation: Current head 202c516 differs from pull request most recent head 5062aee. Consider uploading reports for the commit 5062aee to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #415 +/- ## ======================================= Coverage 97.37% 97.37% ======================================= Files 31 31 Lines 2512 2512 ======================================= Hits 2446 2446 Misses 66 66 ```

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