[rubygems/rubygems] Fix another strange error when running bundle add in frozen mode

If there's a lockfile, but it's out of sync with the Gemfile because a
dependency has been deleted, and frozen mode is set, Bundler will print
the following strange error:

```
$ bundle add rake
, but the lockfile can't be updated because frozen mode is set

You have deleted from the Gemfile:
* rake (~> 13.2)

Run `bundle install` elsewhere and add the updated Gemfile to version control.
```

This commit changes the error to:

```
Some dependencies were deleted from your gemfile, but the lockfile can't be updated because frozen mode is set

You have deleted from the Gemfile:
* rake (~> 13.2)

Run `bundle install` elsewhere and add the updated Gemfile to version control.
```

https://github.com/rubygems/rubygems/commit/452da4048d

lib/bundler/definition.rb

@@ -399,7 +399,7 @@ module Bundler
         changed << "* #{name} from `#{lockfile_source_name}` to `#{gemfile_source_name}`"
       end
 
-      reason = change_reason
+      reason = nothing_changed? ? "some dependencies were deleted from your gemfile" : change_reason
       msg = String.new
       msg << "#{reason.capitalize.strip}, but the lockfile can't be updated because frozen mode is set"
       msg << "\n\nYou have added to the Gemfile:\n" << added.join("\n") if added.any?

spec/bundler/install/deploy_spec.rb

@@ -388,7 +388,7 @@ RSpec.describe "install in deployment or frozen mode" do
       expect(out).not_to include("* myrack-obama")
     end
 
-    it "explodes if you remove a gem and don't check in the lockfile" do
+    it "explodes if you replace a gem and don't check in the lockfile" do
       gemfile <<-G
         source "https://gem.repo1"
         gem "activesupport"
@@ -402,6 +402,17 @@ RSpec.describe "install in deployment or frozen mode" do
       expect(err).not_to include("You have changed in the Gemfile")
     end
 
+    it "explodes if you remove a gem and don't check in the lockfile" do
+      gemfile 'source "https://gem.repo1"'
+
+      bundle "config set --local deployment true"
+      bundle :install, raise_on_error: false
+      expect(err).to include("Some dependencies were deleted")
+      expect(err).to include("frozen mode")
+      expect(err).to include("You have deleted from the Gemfile:\n* myrack")
+      expect(err).not_to include("You have changed in the Gemfile")
+    end
+
     it "explodes if you add a source" do
       gemfile <<-G
         source "https://gem.repo1"