Remove GenericMatrix::ident_zeros

[Prof Garth Wells]

The function is wrong in lots of ways, and it shouldn't even be needed. A code should know which rows are zero and modify with 'set' functions.

[Anders Logg (Chalmers)]

ok to remove as soon as it has been replcaed in MultiMeshAssembler.cpp. It's the only place in the library where it's being used:

find dolfin -name '*.cpp' | xargs grep ident_zeros

[Jan Blechta]

Removal ok for me.