Remove remaining mysql cruft

MySQL support was removed many months ago as we chose PostgreSQL. 

It turns out, these two methods become a lot more readable without all of the switching on adapter type.

:smiley:  :cake:

I chose not to remove the the remaining references to MySQL in [active_record_query_parts.rb](https://github.com/ManageIQ/manageiq/blob/master/vmdb/lib/active_record_query_parts.rb) because that might be useful to be database agnostic elsewhere.  Additionally, I didn't touch the old migrations.
Was more thinking like the concat method...

```ruby
  def self.regexp(field, regex)
    return case ActiveRecord::Base.connection.adapter_name
    when "PostgreSQL" then "#{field} SIMILAR TO '#{regex}'"
    when "MySQL"      then "#{field} REGEXP '#{regex}'"
    else raise NotImplementedError
    end
  end

  def self.not_regexp(*args)
    return case ActiveRecord::Base.connection.adapter_name
    when "PostgreSQL" then "#{field} NOT SIMILAR TO '#{regex}'"
    when "MySQL"      then "#{field} NOT REGEXP '#{regex}'"
    else raise NotImplementedError
    end
  end
end
```

Then...

```ruby
snap_clause  = "AND #{ActiveRecordQueryPartys.not_regexp("base_name", "%\-[0-9][0-9][0-9][0-9][0-9][0-9]\.vmdk")}"
```