cmake: Remove unused variable
cmake: Remove unused variable

DL is empty and unused elsewhere.

The intention at one point may have been to use CMAKE_DL_LIBS, but that
would more likely apply in some situations involving static linking.

eiabea/bitmonero,ranok/bitmonero,ranok/bitmonero,eiabea/bitmonero,eiabea/bitmonero,ranok/bitmonero,ranok/bitmonero,eiabea/bitmonero,eiabea/bitmonero,eiabea/bitmonero,ranok/bitmonero,ranok/bitmonero