Simplify engine internals

[roman-khimov]

This continues #3008 with internal refactoring that is mostly about making code simpler and dropping useless code.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 162 lines in your changes missing coverage. Please review.

Project coverage is 22.83%. Comparing base (6ea74d2) to head (99d4320). Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/local_object_storage/engine/inhume.go	45.31%	33 Missing and 2 partials :warning:
pkg/local_object_storage/engine/container.go	0.00%	31 Missing :warning:
pkg/local_object_storage/engine/select.go	16.66%	19 Missing and 1 partial :warning:
pkg/local_object_storage/engine/lock.go	48.38%	15 Missing and 1 partial :warning:
pkg/local_object_storage/engine/get.go	78.87%	9 Missing and 6 partials :warning:
pkg/local_object_storage/engine/status.go	0.00%	9 Missing :warning:
pkg/local_object_storage/engine/head.go	72.00%	7 Missing :warning:
pkg/local_object_storage/engine/tree.go	0.00%	7 Missing :warning:
pkg/local_object_storage/engine/exists.go	25.00%	5 Missing and 1 partial :warning:
pkg/local_object_storage/engine/put.go	64.28%	4 Missing and 1 partial :warning:
... and 3 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3010 +/- ## ========================================== - Coverage 22.99% 22.83% -0.16% ========================================== Files 790 790 Lines 58500 58342 -158 ========================================== - Hits 13454 13325 -129 + Misses 44167 44136 -31 - Partials 879 881 +2 ```

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[roman-khimov]

Full run: https://github.com/nspcc-dev/neofs-node/actions/runs/11803078321