asserts: remove unused function, fix for linter

[pedronis]

this was flagged by deadcode/unused.

there is no obvious future use case for this anymore so I'm removing it
but this will not always be clear cut with header_checks.go functions
which is a growing helper library

[stolowski]

Thanks

[bboozzoo]

LGTM

[codecov-commenter]

Codecov Report

Merging #11382 (50d83b8) into master (984e5be) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #11382      +/-   ##
==========================================
- Coverage   78.36%   78.36%   -0.01%     
==========================================
  Files         931      931              
  Lines      106644   106642       -2     
==========================================
- Hits        83574    83570       -4     
- Misses      17867    17868       +1     
- Partials     5203     5204       +1     

Flag	Coverage Δ
unittests	78.36% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
asserts/header_checks.go	95.77% <ø> (+0.89%)	⬆️
cmd/snap/cmd_aliases.go	90.14% <0.00%> (-1.41%)	⬇️
cmd/snap/cmd_debug_state.go	70.18% <0.00%> (-0.46%)	⬇️
overlord/ifacestate/handlers.go	64.87% <0.00%> (-0.15%)	⬇️

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[anonymouse64]

Lgtm