Remove unused _handleDropdownTap

Change-Id: Ie4a8effd6d814985cafa6dea9239903f503dae33
(cherry picked from commit 0762766561fdab34bb38b6037ab8040fe65a8bda)

polygerrit-ui/app/elements/shared/gr-dropdown-list/gr-dropdown-list_html.ts

diff --git a/polygerrit-ui/app/elements/shared/gr-dropdown-list/gr-dropdown-list_html.ts b/polygerrit-ui/app/elements/shared/gr-dropdown-list/gr-dropdown-list_html.ts
index 26a6b3f..28eb6a4 100644
--- a/polygerrit-ui/app/elements/shared/gr-dropdown-list/gr-dropdown-list_html.ts
+++ b/polygerrit-ui/app/elements/shared/gr-dropdown-list/gr-dropdown-list_html.ts
@@ -145,7 +145,6 @@
       slot="dropdown-content"
       attr-for-selected="data-value"
       selected="{{value}}"
-      on-tap="_handleDropdownTap"
     >
       <template
         is="dom-repeat"