Remove hierarchy declaration for MysqlAdapter and MysqlAdapter2 classes redefinition

When using this gem in JRuby, we get the following error:
```
TypeError: superclass mismatch for class MysqlAdapter
  ConnectionAdapters at src/trackless_triggers/lib/trackless_triggers.rb:85
        ActiveRecord at src/trackless_triggers/lib/trackless_triggers.rb:41
              (root) at trackless_triggers/lib/trackless_triggers.rb:3
             require at org/jruby/RubyKernel.java:1065
              (root) at .rvm/gems/jruby-1.7.16@global/gems/bundler-1.7.3/lib/bundler/runtime.rb:1
                each at org/jruby/RubyArray.java:1613
             require at .rvm/gems/jruby-1.7.16@global/gems/bundler-1.7.3/lib/bundler/runtime.rb:76
                each at org/jruby/RubyArray.java:1613
             require at .rvm/gems/jruby-1.7.16@global/gems/bundler-1.7.3/lib/bundler/runtime.rb:72
             require at .rvm/gems/jruby-1.7.16@global/gems/bundler-1.7.3/lib/bundler/runtime.rb:61
             require at .rvm/gems/jruby-1.7.16@global/gems/bundler-1.7.3/lib/bundler.rb:133
             require at org/jruby/RubyKernel.java:1065
              (root) at script/rails:6
```
This is caused by the fact that the MysqlAdapter class defined in the activerecord-jdbc-adapter has different hierarchy:
https://github.com/jruby/activerecord-jdbc-adapter/blob/bfab971843d4abd9a818c539645856df021c2c2f/lib/arjdbc/mysql/adapter.rb#L614

This PR fixes this problem.