docs: remove demo-app docs

[ajcasagrande]

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Please check if your PR fulfills the following requirements:

• I am not introducing a breaking change (if you are, flag in conventional commit message with BREAKING CHANGE: describing the break)
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[codecov-commenter]

Codecov Report

Merging #273 (ee4e5af) into main (35e0b63) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##             main     #273   +/-   ##
=======================================
  Coverage   33.53%   33.53%           
=======================================
  Files          18       18           
  Lines        1792     1792           
=======================================
  Hits          601      601           
  Misses       1125     1125           
  Partials       66       66           

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[presatish]

README.md file referred in some places and the links to it are broken for some reason. For e.g. https://github.com/EdgeX-Camera-Management/device-onvif-camera/blob/remove-demo-app-docs/doc/general-usage.md#next-steps

[presatish]

Need some minor changes.

[ChristianDarr-personal]

Same comment as other people, what do we want to do with the files left dangling from removing this line?

[presatish]

I think for now we can leave these files as it is at least until we migrate everything to edgex docs.