docs(Dialog): remove cancel button in an example containing X

[lucivpav]

Both X and cancel button cannot coexist per design guidelines.

Don't forget to close the related VSTS issue once this is merged.

[layershifter]

Stardust is not a guideline to MS Teams.
I want to avoid merging this as it's very frequent thing in Web.

[codecov]

Codecov Report

Merging #1949 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1949   +/-   ##
======================================
  Coverage    70.4%   70.4%           
======================================
  Files         892     892           
  Lines        7870    7870           
  Branches     2298    2274   -24     
======================================
  Hits         5541    5541           
  Misses       2316    2316           
  Partials       13      13

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