Don't suggest `--full-index` on API Response mismatch errors

I've never seen this error in real life, and if it was happening, I
think it's either some server side issue that would need to be fixed or
some transient issue. We should move away from the full index, since
it's slow, so let's stop recommending it.

lib/bundler/shared_helpers.rb

@@ -160,7 +160,7 @@ module Bundler
         " (was expecting #{old_deps.map(&:to_s)}, but the real spec has #{new_deps.map(&:to_s)})"
       raise APIResponseMismatchError,
         "Downloading #{spec.full_name} revealed dependencies not in the API or the lockfile (#{extra_deps.join(", ")})." \
-        "\nEither installing with `--full-index` or running `bundle update #{spec.name}` should fix the problem."
+        "\nRunning `bundle update #{spec.name}` should fix the problem."
     end
 
     def pretty_dependency(dep)

spec/bundler/install/gems/compact_index_spec.rb

@@ -930,7 +930,7 @@ The checksum of /versions does not match the checksum provided by the server! So
     expect(out).to include("rails-2.3.2 from rubygems remote at #{source_uri}/ has either corrupted API or lockfile dependencies")
     expect(err).to include(<<-E.strip)
 Bundler::APIResponseMismatchError: Downloading rails-2.3.2 revealed dependencies not in the API or the lockfile (#{deps.map(&:to_s).join(", ")}).
-Either installing with `--full-index` or running `bundle update rails` should fix the problem.
+Running `bundle update rails` should fix the problem.
     E
   end
 

spec/bundler/lock/lockfile_spec.rb

@@ -1218,7 +1218,7 @@ RSpec.describe "the lockfile format" do
     G
 
     expect(err).to include("Downloading rack_middleware-1.0 revealed dependencies not in the API or the lockfile (#{Gem::Dependency.new("rack", "= 0.9.1")}).").
-      and include("Either installing with `--full-index` or running `bundle update rack_middleware` should fix the problem.")
+      and include("Running `bundle update rack_middleware` should fix the problem.")
   end
 
   it "regenerates a lockfile with no specs" do