Can I just delete an asset from my farm asset summary when I don't have it anymore?

Unfortunately, you do not want to delete the asset without reporting how it was disposed of.  You are required to maintain an accurate Asset Summary until the asset is reported as disposed.  There could be a liability for the recapture of depreciation when you dispose of an asset for your farm.  You can report the disposal in TurboTax by selecting how the asset was disposed of (sold, converted to personal use, etc..) in your Asset Summary.  

Follow these steps to mark an asset as disposed of:

1. Select Edit beside the asset 
2. Select Sold/disposed of in the Asset Summary list

Here is a link on managing assets:

Managing Assets