cleanup: use SliceBuffer directly where no pool is available

[ash2k]

As discussed in #7653 (comment), this is a minor code simplification.

RELEASE NOTES: none

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.91%. Comparing base (60c70a4) to head (c5dc2b1).
Report is 13 commits behind head on master.

Files with missing lines	Patch %	Lines
codec.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7827      +/-   ##
==========================================
- Coverage   81.93%   81.91%   -0.02%     
==========================================
  Files         373      373              
  Lines       37880    37880              
==========================================
- Hits        31036    31031       -5     
- Misses       5550     5559       +9     
+ Partials     1294     1290       -4     

Files with missing lines	Coverage Δ
preloader.go	58.33% <100.00%> (ø)
rpc_util.go	79.48% <100.00%> (ø)
codec.go	9.09% <0.00%> (ø)

... and 17 files with indirect coverage changes

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[easwars]

Moving to @dfawley for second set of eyes.

[dfawley]

Thanks!