removed useless abstract class

[qiuxd]

We could use default method in interface instead of creating an abstract class that does the same thing.

[coveralls]

Coverage increased (+0.03%) to 86.468% when pulling 3a36886 on qiuxd:simplified into 0b0d024 on ServiceComb:master.

[WillemJiang]

Thanks for the PR, as the AbstractHandler may be used by user, we cannot just remove this class simply without any warning message.

My suggestion is we just keep the class there and add the Deprecated annotation to let the user know about it, and then we can remove it in the next release.

[coveralls]

Coverage increased (+0.04%) to 86.478% when pulling c5f023e on qiuxd:simplified into 0b0d024 on ServiceComb:master.