fix(vscode): fix behaviour when only 1 generator is available & refactor old code

[MaxKless]

• lets user select generator when there is only one generator instead of automatically selecting it (this won't affect the move/remove actions)
• fixes generatorName data that is used in generate ui header
• show proper error when no move / remove generators are available
• simplifies & refactors old code (parts of this were still left over from when there was still a task & generate ui)

[nx-cloud]

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[jaysoo]

LGTM