Remove incorrect comment stating goog.date.Date is marked as extending Date.
Remove incorrect comment stating goog.date.Date is marked as extending Date.

RELNOTES: n/a

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146018735

redforks/closure-library,lucidsoftware/closure-library,redforks/closure-library,google/closure-library,google/closure-library,redforks/closure-library,lucidsoftware/closure-library,lucidsoftware/closure-library,ikabirov/closure-library,teppeis/closure-library,platinumpixs/google-closure-library,ikabirov/closure-library,teppeis/closure-library,teppeis/closure-library,ikabirov/closure-library,platinumpixs/google-closure-library,lucidsoftware/closure-library,google/closure-library,platinumpixs/google-closure-library,redforks/closure-library,platinumpixs/google-closure-library,ikabirov/closure-library,google/closure-library,google/closure-library,teppeis/closure-library