[5.6][build] Clean up ICU detritus that is no longer used to build the stdlib #40817
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Cherrypick of #40625, #40659, and #40774
Explanation: #40340 removed ICU as a dependency for the stdlib before the 5.6 branch, but we weren't able to clean up some of this build configuration that still refers to it before the branch. This pull gets rid of those last ICU references that we no longer use, so nobody gets confused when Swift 5.6 is released.
Scope: This is all build-related and unused on any officially supported platform since @Azoy removed the stdlib dependency on ICU.
SR Issue: None
Risk: Low, mostly doc and some unused build configuration variables and logic.
Testing: Trunk has not had any problems since these were merged.
Reviewer: @compnerd