Fix placeholders position in LineEdit when editing inside the Editor

Editing the `Text` property through the editor causes a wrong
placement of the placeholder, as it calls `LineEdit::clear_internal`,
which was wrongly reseting the cached placeholder width.

Fix #18184.

(cherry picked from commit c17de1f70f)

scene/gui/line_edit.cpp

@@ -1135,7 +1135,6 @@ void LineEdit::clear_internal() {
 
 	_clear_undo_stack();
 	cached_width = 0;
-	cached_placeholder_width = 0;
 	cursor_pos = 0;
 	window_pos = 0;
 	undo_text = "";