Remove redundant zeroclipboard-rails gem

And associated JavaScript config.

This was originally added in this PR [1] and I can't see any other
remnants.

This seems like a good thing, because the functionality it was supposed
to provide wasn't working for me (at least in Firefox) and the GitHub
repo [2] has been archived since 2021!

We're now using the `copy_to_clipboard` component [3] instead.

[1]: #248
[2]: https://github.com/zeroclipboard/zeroclipboard-rails
[3]: https://components.publishing.service.gov.uk/component-guide/copy_to_clipboard

Gemfile

@@ -32,7 +32,6 @@ gem "sprockets-rails"
 gem "uglifier"
 gem "uuid"
 gem "whenever"
-gem "zeroclipboard-rails"
 
 # GDS Gems
 gem "gds-api-adapters"

Gemfile.lock

@@ -689,8 +689,6 @@ GEM
     xpath (3.2.0)
       nokogiri (~> 1.8)
     zeitwerk (2.6.12)
-    zeroclipboard-rails (0.1.2)
-      railties (>= 3.1)
     zxcvbn-ruby (1.2.0)
 
 PLATFORMS
@@ -754,7 +752,6 @@ DEPENDENCIES
   uuid
   webmock
   whenever
-  zeroclipboard-rails
 
 BUNDLED WITH
    2.4.10

app/assets/javascripts/legacy_layout.js

@@ -1,3 +1,2 @@
 //= require chosen.jquery
 //= require_directory ./legacy/modules
-//= require zeroclipboard