pkg/installation: unused pluginVersionFromPath

[ahmetb]

Removing unused method.

Aside: It would be good to have a lint check that validates that unexposed
methods are not used (maybe even for external methods).

/kind cleanup
/assign @corneliusweig

[corneliusweig]

/lgtm

[codecov-io]

Codecov Report

Merging #290 into master will increase coverage by 1.38%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #290      +/-   ##
==========================================
+ Coverage   54.84%   56.22%   +1.38%     
==========================================
  Files          19       19              
  Lines         877      891      +14     
==========================================
+ Hits          481      501      +20     
+ Misses        342      337       -5     
+ Partials       54       53       -1

Impacted Files	Coverage Δ
pkg/installation/util.go	37.5% <ø> (-3.88%)	⬇️
pkg/index/platform.go
pkg/installation/platform.go	91.66% <0%> (ø)
pkg/index/validation/validate.go	93.65% <0%> (+2.95%)	⬆️

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