`HoistVariablesAnalysis`: remove unused explicit interfaces after inlining

[awnawab]

This small PR implements two bug fixes:

• HoistVariablesAnalysis now skips successors that might no longer be present (e.g. they were inlined)
• Unused explicit interfaces of inlined subroutines are now removed

[github-actions[bot]]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/319/index.html

[codecov[bot]]

Codecov Report

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

Project coverage is 95.16%. Comparing base (eb793e2) to head (b9132db).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #319 +/- ## ========================================== + Coverage 95.15% 95.16% +0.01% ========================================== Files 168 168 Lines 35454 35538 +84 ========================================== + Hits 33737 33821 +84 Misses 1717 1717 ``` | [Flag](https://app.codecov.io/gh/ecmwf-ifs/loki/pull/319/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ecmwf-ifs) | Coverage Δ | | |---|---|---| | [lint_rules](https://app.codecov.io/gh/ecmwf-ifs/loki/pull/319/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ecmwf-ifs) | `96.38% <ø> (ø)` | | | [loki](https://app.codecov.io/gh/ecmwf-ifs/loki/pull/319/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ecmwf-ifs) | `95.14% <100.00%> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ecmwf-ifs#carryforward-flags-in-the-pull-request-comment) to find out more.

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[awnawab]

Hi. Can I push a small update here?