project panel: Fix entries being marked when switching between tabs (#20596)

This is a follow-up (or related to) #20412.

It fixes entries being marked when navigating between tabs with `cmd-[`
and `cmd-]`.

Turns out that deep in the bowels of the project panel, we check whether
a `shift` modifier was pressed - which is the case with `cmd-[` on a US
ANSI layout - and if so mark an entry.

I think that's a left-over, because all the other code paths that
select/reveal an entry mark it explicitly too.

Release Notes:

- Fixed entries in project panel being marked when navigating between
tabs with keybinding that uses `shift` modifier.

crates/project_panel/src/project_panel.rs

@@ -2145,12 +2145,6 @@ impl ProjectPanel {
                 worktree_id,
                 entry_id,
             });
-            if cx.modifiers().shift {
-                self.marked_entries.insert(SelectedEntry {
-                    worktree_id,
-                    entry_id,
-                });
-            }
         }
     }