[flang][NFC] Remove obsolete Character helper

During the upstreaming process from fir-dev some
new builder have been introduced in the `flang/Optimizer/Builder/Character.h`
and `flang/include/Builder/Runtime/Character.h` files.
This patch removes the obsolete Charachter helpers still present
in the lowering directories.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D118594