fix(tabline): avoid memory leak in tabline click definitions (#21847)

Problem:    Memory is leaked in tabline click definitions since
            https://github.com/neovim/neovim/pull/21008.
Solution:   Add back a call to `stl_clear_click_defs()` that was lost in
            the refactor PR.

test/functional/ui/tabline_spec.lua

@@ -119,4 +119,10 @@ describe("tabline", function()
       [2] = {bold = true, foreground = Screen.colors.Blue};
     }}
   end)
+
+  it('click definitions do not leak memory #21765', function()
+    command('set tabline=%@MyClickFunc@MyClickText%T')
+    command('set showtabline=2')
+    command('redrawtabline')
+  end)
 end)