[ISSUE #7970]remove unnecessary type cast

[ChineseTony]

Which Issue(s) This PR Fixes

Fixes #7970

Brief Description

How Did You Test This Change?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 42.83%. Comparing base (59220d8) to head (223de46).

Files	Patch %	Lines
...ting/protocol/header/SendMessageRequestHeader.java	0.00%	2 Missing ⚠️
...che/rocketmq/remoting/netty/NettyServerConfig.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #7971      +/-   ##
=============================================
- Coverage      42.85%   42.83%   -0.02%     
+ Complexity     10353    10344       -9     
=============================================
  Files           1270     1270              
  Lines          88619    88617       -2     
  Branches       11395    11395              
=============================================
- Hits           37975    37958      -17     
- Misses         45962    45968       +6     
- Partials        4682     4691       +9     

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[ChineseTony]

hello @RongtongJin @lizhimins request.decodeCommandCustomHeader have generic type, Is it necessary remove all redundancy code for type case?