You don't have to delete them, just don't use them. Start your render at the frame you want. if it's an animation, just start the animation at frame 30 to the end. If it's a still image, just render the frame you want. If you delete them, your cloth simulation won't work since it is set up to start at the same place no matter what you do. If you remove the first 29 frames, your cloth simulation will still start at what used to be frame 1 not what used to be frame 30.

But to answer your question anyway, open the animation pallet and select the key frames starting at frame 30 all the way to the end, and then drag all the key frames back to frame 1. Then just delete the left over frames. You can also do it with "Animation/Retime animation". Source frames 30 to X. Destination frames: 1-X

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