Open Preview appears twice when adding the "vscode-builtin-markdown" plugins

[lmcbout]

Open Preview appears twice:

• under the context menu
• under "Open With" sub-menu

Note: Select any README.md file under Theia

This comes from the following built-in plugins:

• vscode-builtin-markdown

Do we need to keep this build-in or we just use Theia without it and have only one option under "Open with/Preview" for a markdown file ?

[vince-fugnitto]

The issue has been tracked previously: #6984.
There is no harm in keeping both, once it is fully supported @theia/preview will be dropped.