roundrobin: Remove unnecessary ExitIdle override

[arjan-bal]

This override was introduced recently in #8367. It's not necessary.

RELEASE NOTES: N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.33%. Comparing base (d2e8366) to head (d680c30).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8390      +/-   ##
==========================================
+ Coverage   82.17%   82.33%   +0.15%     
==========================================
  Files         419      419              
  Lines       42065    42063       -2     
==========================================
+ Hits        34567    34631      +64     
+ Misses       6023     5980      -43     
+ Partials     1475     1452      -23     

Files with missing lines	Coverage Δ
balancer/roundrobin/roundrobin.go	100.00% <ø> (ø)

... and 21 files with indirect coverage changes

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