Fix multicursors not being added when clicking on line numbers (#28263)

Closes https://github.com/zed-industries/zed/issues/21372

Release Notes:

- Fixed multicursors not being added when clicking on line numbers

-----

I tracked this down to
b6ee367ee0/crates/editor/src/element.rs (L591)

being forwarded to `editor.select()` a few lines below
b6ee367ee0/crates/editor/src/element.rs (L667-L675)

resulting in `add == true` and `click_count == 3`, triggering this
b6ee367ee0/crates/editor/src/editor.rs (L2750-L2752)

... and we end up removing the previous selection. 

This was added [in
2021](bfecdb7bc0)
under this reasoning:

> This prevents selections added in earlier clicks from being rendered
under the pending selection.

which no longer seems to be an issue, so removing should be safe?

---------

Co-authored-by: Peter Tripp <peter@zed.dev>

crates/editor/src/editor.rs

@@ -2972,7 +2972,6 @@ impl Editor {
 
         let display_map = self.display_map.update(cx, |map, cx| map.snapshot(cx));
         let buffer = &display_map.buffer_snapshot;
-        let newest_selection = self.selections.newest_anchor().clone();
         let position = display_map.clip_point(position, Bias::Left);
 
         let start;
@@ -3047,8 +3046,6 @@ impl Editor {
             } else {
                 if !add {
                     s.clear_disjoint();
-                } else if click_count > 1 {
-                    s.delete(newest_selection.id)
                 }
 
                 s.set_pending_anchor_range(start..end, mode);