[rubygems/rubygems] Remove unreachable code

Nothing is actually raising this at the moment.

https://github.com/rubygems/rubygems/commit/3b824ca7a6

lib/rubygems/commands/install_command.rb

@@ -224,10 +224,6 @@ You can use `i` command instead of `install`.
       rescue Gem::InstallError => e
         alert_error "Error installing #{gem_name}:\n\t#{e.message}"
         exit_code |= 1
-      rescue Gem::GemNotFoundException => e
-        show_lookup_failure e.name, e.version, e.errors, suppress_suggestions
-
-        exit_code |= 2
       rescue Gem::UnsatisfiableDependencyError => e
         show_lookup_failure e.name, e.version, e.errors, suppress_suggestions,
                             "'#{gem_name}' (#{gem_version})"