Deleting multiple unsynced downloaded music on my iphone through itunes on my windows PC

Hello apple,

I have my own downloaded music on my windows PC, I wanted to copy them to my Iphone and hence i used itunes to copy the music to my iphone.

after some period, I have removed all the music on my PC and i have synced my iphone again with the intention to remove all the music on my iphone.

Issue: I can still see the music files unhighlighted and when clicked on the music files it says, 'This item is not synced across your devices'.

I don't want these music files to appear on my music list. I want them to be removed. I want to sync new music again.

How do i remove these unhighlighted music files? I have several thousands of unhighlighted music files to remove.