fix: remove FixedSizeArena

[ShiKaiWi]

Which issue does this PR close?

Closes #762

Rationale for this change

The FixedSizeArena is just a demo whose correctness and unit tests are missing, and it may make people confusing. So let's remove it.

What changes are included in this PR?

Remove the FixedSizeArena.

Are there any user-facing changes?

None.

How does this change test

No need to test.

[codecov-commenter]

Codecov Report

Merging #772 (36bd6e6) into main (50471c0) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

❗ Current head 36bd6e6 differs from pull request most recent head 961a386. Consider uploading reports for the commit 961a386 to get more accurate results

@@            Coverage Diff             @@
##             main     #772      +/-   ##
==========================================
- Coverage   68.48%   68.41%   -0.08%     
==========================================
  Files         296      295       -1     
  Lines       46648    46592      -56     
==========================================
- Hits        31947    31876      -71     
- Misses      14701    14716      +15     

Impacted Files	Coverage Δ
components/arena/src/lib.rs	100.00% <ø> (ø)
analytic_engine/src/instance/read.rs	77.85% <100.00%> (-0.31%)	⬇️
analytic_engine/src/row_iter/mod.rs	100.00% <100.00%> (ø)

... and 8 files with indirect coverage changes

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[jiacai2050]

LGTM