[clang codegen] Delete unnecessary GEP cleanup code. (#90303)

There's some code in AggExprEmitter::VisitCXXParenListOrInitListExpr to
try to do early cleanup for GEPs for fields that aren't accessed. But
it's unlikely to actually save significant compile-time, and it's subtly
wrong in cases where EmitLValueForFieldInitialization() doesn't create a
GEP. So just delete the code.

Fixes #88077. Fixes #89547.