Remove check workflow & improve DCO check script

[Warashi]

What this PR does:

• Remove non-maintainer check workflow
• Ignore merge commit in the check/dco

Why we need it:

• It's a bit complex compared to its benefit
• merge commits may lack sign-off, and it's allowed

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 27.89%. Comparing base (29e3ccf) to head (9424489).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5904      +/-   ##
==========================================
- Coverage   27.93%   27.89%   -0.05%     
==========================================
  Files         516      519       +3     
  Lines       55450    55923     +473     
==========================================
+ Hits        15492    15600     +108     
- Misses      38767    39125     +358     
- Partials     1191     1198       +7     

Flag	Coverage Δ
.	23.75% <ø> (ø)
.-pkg-app-pipedv1-plugin-example	0.00% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	63.32% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	64.93% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-plugin-sdk	49.25% <ø> (ø)
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	22.83% <ø> (?)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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[khanhtc1202]

Thanks 👍