remove link to old externalGradingAws.md docs
remove link to old externalGradingAws.md docs

mwest1066/PrairieLearn,parasgithub/PrairieLearn,parasgithub/PrairieLearn,rbessick5/PrairieLearn,mwest1066/PrairieLearn,mwest1066/PrairieLearn,mwest1066/PrairieLearn,rbessick5/PrairieLearn,rbessick5/PrairieLearn,parasgithub/PrairieLearn,rbessick5/PrairieLearn,mwest1066/PrairieLearn,parasgithub/PrairieLearn,rbessick5/PrairieLearn,parasgithub/PrairieLearn