cdt-gdb: update repo to consume 'cdt-gdb-vscode' from open-vsx

[vince-fugnitto]

Description

The repository should be updated to consume cdt-gdb-vscode from open-vsx instead of how we do it currently by listing it under adapters and using the download-debug-adapters.js script:

theia-cpp-extensions/packages/cpp-debug/package.json

Lines 56 to 58 in bb6bb9d

	"adapters": {
	"cdt-gdb-vscode": "https://github.com/theia-ide/cdt-gdb-vscode/releases/download/v0.0.90-theia.2/cdt-gdb-vscode-0.0.90-theia.2.tgz"
	}

The download-debug-adapters.js script should be removed in favor of downloading plugins through the theiaPlugins mechanism in the package.json: eclipse-theia/theia#8713.

[colin-grant-work]

This was done as part of #119:

theia-cpp-extensions/package.json

Line 80 in 53b9061

"cdt-gdb-vscode": "https://open-vsx.org/api/eclipse-cdt/cdt-gdb-vscode/0.0.92/file/eclipse-cdt.cdt-gdb-vscode-0.0.92.vsix"