[chore] fix nits in persistent queue implementation

[bogdandrutu]

• Remove unused members
• Fix tests assert vs require, and usage of Eventually when not needed.

[codecov]

Codecov Report

Base: 90.99% // Head: 90.99% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (ebaae1a) compared to base (8bf3885).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7203      +/-   ##
==========================================
- Coverage   90.99%   90.99%   -0.01%     
==========================================
  Files         293      293              
  Lines       14271    14267       -4     
==========================================
- Hits        12986    12982       -4     
  Misses       1017     1017              
  Partials      268      268              

Impacted Files	Coverage Δ
...porter/exporterhelper/internal/persistent_queue.go	100.00% <100.00%> (ø)

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