BREAKING CHANGE: remove dubbo protocol impl which replaced by the impl in dubbo-go project

[wongoo]

What this PR does:
The implementation of dubbo2 rpc protocol in dubbo-go-hessian2 project is deprecated for a long time.

The dubbo team has already maintained the implementation in dubbo-go project instead.

To avoid confusion for users, it's better to remove dubbo protocol impl in dubbo-go-hessian2.

Which issue(s) this PR fixes:

NONE

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Merging #364 (53c2f20) into master (f0fbe40) will increase coverage by 2.42%.
The diff coverage is n/a.

❗ Current head 53c2f20 differs from pull request most recent head f862307. Consider uploading reports for the commit f862307 to get more accurate results

@@            Coverage Diff             @@
##           master     #364      +/-   ##
==========================================
+ Coverage   68.98%   71.40%   +2.42%     
==========================================
  Files          28       25       -3     
  Lines        3140     2616     -524     
==========================================
- Hits         2166     1868     -298     
+ Misses        751      591     -160     
+ Partials      223      157      -66     

see 2 files with indirect coverage changes

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