Remove redundant code

[youngxhui]

Remove redundant code

engine.RouterGroup.engine = engine

the RouterGroup can remove.

[codecov]

Codecov Report

Merging #3606 (8d62643) into master (6a0556e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3606   +/-   ##
=======================================
  Coverage   99.01%   99.01%           
=======================================
  Files          42       42           
  Lines        3151     3151           
=======================================
  Hits         3120     3120           
  Misses         21       21           
  Partials       10       10           

Flag	Coverage Δ
	99.01% <100.00%> (ø)
go-1.18	98.92% <100.00%> (ø)
go-1.19	99.01% <100.00%> (ø)
go-1.20	99.01% <100.00%> (ø)
macos-latest	?
ubuntu-latest	99.01% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
gin.go	99.19% <100.00%> (ø)

[appleboy]

build fail