Place the cursor on the line you want to delete
Select the line by dragging across it or triple-clicking it
Press Backspace or Delete
If it is a paragraph line, place the cursor anywhere in the paragraph and press Backspace or Delete
If it is a horizontal line, select it and press Delete
If it is a table row, right-click the row and choose Delete Rows
If it is a text box line, click the border and press Delete