I click the SHIFT key at the same time as CTRL+A, so the keyboard shortcut is therefore SHIFT+CTRL+A to select all trendlines. Afterwards, I press DELETE to remove them all.
By clicking CTRL+A, this shortcut selects every component added within the price chart (indicators, trendlines, tools, etc.). If you have indicators such as RSI added to your price chart, it creates a smaller separate window within the same original chart area. CTRL+A does not select these separate windows. The shortcut only selects indicators and components within the chart, such as moving averages, trendlines, fibonacci levels, etc.
I hope this simplifies the shortcuts