remove the unnecessary file localization steps when karmadactl register

[zhzhuang-zju]

What type of PR is this?
/kind feature

What this PR does / why we need it:
When executing the karmadactl register command, there are many localized file operations involved, which lead to the following issues:

• The current file storage path is /etc/karmada, and non-admin users will not have permission to create this directory. see #test/e2e/karmadactl_test.go: test register command #5334 (comment)
• Sensitive information such as CA should not be stored in files
• When karmadactl register fails, the residual files can cause the next execution of karmadactl register to fail.

In fact, the file localization steps in karmadactl register are unnecessary and can be removed.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
If we want to use karmadactl register in CI to join a pull mode cluster, it will fail due to lack of root permissions.
https://github.com/karmada-io/karmada/actions/runs/11608077257/job/32322805939?pr=5763#step:5:934

Does this PR introduce a user-facing change?:

[karmada-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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• pkg/karmadactl/OWNERS

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[zhzhuang-zju]

cc @mohamedawnallah

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 46.38%. Comparing base (079d0ab) to head (4301b08).
Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/karmadactl/register/register.go	0.00%	9 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5862      +/-   ##
==========================================
+ Coverage   46.31%   46.38%   +0.07%     
==========================================
  Files         661      661              
  Lines       54364    54339      -25     
==========================================
+ Hits        25177    25207      +30     
+ Misses      27562    27508      -54     
+ Partials     1625     1624       -1     

Flag	Coverage Δ
unittests	46.38% <0.00%> (+0.07%)	⬆️

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