[flang][NFC] Remove obsolete ComplexExpr helper

Functionality present in `flang/include/flang/Lower/ComplexExpr.h` are
available in `flang/include/flang/Optimizer/Builder/Complex.h`. This patch removes
the obsolete files.

Reviewed By: kiranchandramohan, schweitz

Differential Revision: https://reviews.llvm.org/D118462