Remove duplicated BuildConfigFromFlags func

[antoineco]

What this PR does / why we need it:

Use BuildConfigFromFlags from k8s.io/client-go/tools/clientcmd instead of local, identical copy.

[k8s-reviewable]

This change is 

[aledbf]

/approve

[aledbf]

@antoineco thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, antoineco

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[codecov-io]

Codecov Report

Merging #2162 into master will decrease coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2162      +/-   ##
==========================================
- Coverage   36.59%   36.44%   -0.15%     
==========================================
  Files          69       69              
  Lines        4875     4862      -13     
==========================================
- Hits         1784     1772      -12     
+ Misses       2821     2820       -1     
  Partials      270      270

Impacted Files	Coverage Δ
cmd/nginx/main.go	21.37% <100%> (-5.84%)	⬇️

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