[flang][NFC] Remove obsolete FIRBuilder and BoxValue

This patch removes some files made obsolete by newer version
of them available in the Optimizer directory.
`flang/include/flang/Lower/FIRBuilder.h` and `flang/lib/Lower/FIRBuilder.cpp` are
removed and replace by the newer version present in
`flang/include/flang/Optimizer/Builder/FIRBuilder.h` and
`flang/lib/Optimizer/Builder/FIRBuilder.cpp`.
`flang/include/flang/Lower/Support/BoxValue.h` and `flang/lib/Lower/ConvertExpr.cpp` are removed and replace by the newer
version present in `flang/include/flang/Optimizer/Builder/BoxValue.h`

This patch is a preparation to be able to upstream the lowering from
fir-dev.

Reviewed By: jeanPerier, kiranchandramohan

Differential Revision: https://reviews.llvm.org/D118404