Remove unusable --broker flag from trigger update cmd

[dsimansk]

Description

Changes

• Remove unusable --broker flag from trigger update cmd

Reference

Fixes #1813

Release Note

Remove unusable `--broker` flag from `trigger update` cmd

/cc @rhuss @vyasgun

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.03% ⚠️

Comparison is base (8dc8a9f) 79.67% compared to head (cf296dc) 79.64%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1847      +/-   ##
==========================================
- Coverage   79.67%   79.64%   -0.03%     
==========================================
  Files         179      179              
  Lines       13938    13938              
==========================================
- Hits        11105    11101       -4     
- Misses       2067     2070       +3     
- Partials      766      767       +1     

Files Changed	Coverage Δ
pkg/kn/commands/trigger/update_flags.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[rhuss]

/approve
/lgtm

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dsimansk, rhuss

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