Deprecate `DeviceBase::GetStepAllocator()` and replace with calls to `GetAllocator()`.

Deprecate `DeviceBase::GetStepAllocator()` and replace with calls to `GetAllocator()`.

The `GetStepAllocator()` API relied on the existence of a "step resource manager",
which is no longer a concept in the runtime (it was replaced by "step containers").
Since the additional flexibility does not appear to be used in the codebase, the
`GetScopedAllocator()` seems to provide a similar extension point (based on step IDs),
and the `OpKernelContext::get_allocator()` method is called frequently, this change
simplifies the implementation somewhat.

The `GetStepAllocator()` method is retained as a non-virtual stub that forwards to
`GetAllocator()`, because at least one third-party library (libxsmm) calls this
interface directly.

PiperOrigin-RevId: 197922154