refactor: remove encoder method from WalManager

[ShiKaiWi]

Which issue does this PR close?

Closes #

Rationale for this change

Currently, the default implementation of encoder method in WalManager trait is useless because callers can use the LogBatchEncoder directly to create such encoder.

What changes are included in this PR?

Remove the encode method defined in WalManager.

Are there any user-facing changes?

None.

How does this change test

Existing tests.

[codecov-commenter]

Codecov Report

Merging #661 (1cb9240) into main (ce9fa87) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

❗ Current head 1cb9240 differs from pull request most recent head cae57df. Consider uploading reports for the commit cae57df to get more accurate results

@@            Coverage Diff             @@
##             main     #661      +/-   ##
==========================================
- Coverage   66.94%   66.91%   -0.03%     
==========================================
  Files         283      283              
  Lines       43969    43940      -29     
==========================================
- Hits        29435    29404      -31     
- Misses      14534    14536       +2     

Impacted Files	Coverage Δ
analytic_engine/src/instance/engine.rs	64.03% <ø> (ø)
wal/src/lib.rs	100.00% <ø> (ø)
wal/src/manager.rs	90.43% <ø> (-0.12%)	⬇️
analytic_engine/src/instance/alter.rs	95.19% <100.00%> (-0.28%)	⬇️
analytic_engine/src/instance/write.rs	87.05% <100.00%> (-0.32%)	⬇️
analytic_engine/src/manifest/details.rs	95.93% <100.00%> (-0.03%)	⬇️
components/skiplist/src/list.rs	98.87% <0.00%> (-0.23%)	⬇️
wal/src/rocks_impl/manager.rs	92.53% <0.00%> (-0.19%)	⬇️

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[Rachelint]

LGTM