fix: unregister-member deletes secret & has needed permissions

[MatousJobanek]

KUBESAW-247
unregister-member should delete also the ToolchainCluster secret.
the role defines the needed permissions

[MatousJobanek]

I know the list is not needed, but it's technically the same as get so it's fine to keep it there

[alexeykazakov]

Thanks!

[codecov]

Codecov Report

Attention: Patch coverage is 64.70588% with 6 lines in your changes missing coverage. Please review.

Project coverage is 67.80%. Comparing base (ec701fa) to head (325af8b).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/cmd/adm/unregister_member.go	64.70%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
- Coverage   67.83%   67.80%   -0.03%     
==========================================
  Files          44       44              
  Lines        3264     3280      +16     
==========================================
+ Hits         2214     2224      +10     
- Misses        853      857       +4     
- Partials      197      199       +2     

Files with missing lines	Coverage Δ
pkg/cmd/adm/unregister_member.go	56.66% <64.70%> (+2.12%)	⬆️