Tiny cleanup: Remove unused using odb::dbSigType declaration
Tiny cleanup: Remove unused using odb::dbSigType declaration

(https://clang.llvm.org/extra/clang-tidy/checks/misc/unused-using-decls.html)

Signed-off-by: Henner Zeller <e393654266408b751fd8ad27b99ca78813d38e23@google.com>

The-OpenROAD-Project/OpenROAD,QuantamHD/OpenROAD,The-OpenROAD-Project/OpenROAD,QuantamHD/OpenROAD,QuantamHD/OpenROAD,The-OpenROAD-Project/OpenROAD,The-OpenROAD-Project/OpenROAD,QuantamHD/OpenROAD,QuantamHD/OpenROAD,The-OpenROAD-Project/OpenROAD