Don't write vstart in set_vl

All callers (the vector_unit constructor and the vsetvl instructions)
already initialize or otherwise zero vstart.

riscv/vector_unit.cc

@@ -89,7 +89,6 @@ reg_t vectorUnit_t::vectorUnit_t::set_vl(int rd, int rs1, reg_t reqVL, reg_t new
     vl->write_raw(std::min(reqVL, vlmax));
   }
 
-  vstart->write_raw(0);
   return vl->read();
 }