Removing the public member function data() of dpct::device_vector when DPCT_USM_LEVEL_NONE is defined.

dpct::device_vector has two different implementations, one where USM is available and another when USM is not available which that uses sycl::buffer internally for its device memory. This change only effects the implementation when USM is not available and sycl::buffer is used for the memory allocation.

This data() member function does not make a lot of sense to provide when we are using sycl::buffer as the memory backing for dpct::device_vector. Prior to this PR, it provided a pointer into the virtual memory space used to track and look up buffer allocations. Using this returned virtual pointer (when DPCT_USM_LEVEL_NONE is defined) to do things like memcpy() or as input to an algorithm would result in a segfault. It's availability in the API only encourages incorrect usage.

Getting a pointer to the actual device memory is not supported in any viable way by the SYCL specification when a sycl::buffer is the backing memory for a dpct::device_vector. The public member function get_buffer() is provided in this case, and can be used for a similar purpose, but without encouraging incorrect usage.