[flang] Fix -Wunused-but-set-variable in Bridge.cpp (NFC)

llvm-project/flang/lib/Lower/Bridge.cpp:3775:14:
error: variable 'nbDeviceResidentObject' set but not used [-Werror,-Wunused-but-set-variable]
    unsigned nbDeviceResidentObject = 0;
             ^
1 error generated.