[C++][Compute] Remove redundant cast kernels

### Describe the enhancement requested

In Arrow's `cpp/src/arrow/compute/kernels/scalar_cast_string.cc`, there are several cast kernels registered for cases where the input and output types are the same, for example:

https://github.com/apache/arrow/blob/73742e82ed771cb7df40789882e220118045123b/cpp/src/arrow/compute/kernels/scalar_cast_string.cc#L643-L658
https://github.com/apache/arrow/blob/73742e82ed771cb7df40789882e220118045123b/cpp/src/arrow/compute/kernels/scalar_cast_string.cc#L472-L487
https://github.com/apache/arrow/blob/73742e82ed771cb7df40789882e220118045123b/cpp/src/arrow/compute/kernels/scalar_cast_string.cc#L772-L780

However, casts between identical types appear to be handled earlier in cast.cc:

https://github.com/apache/arrow/blob/73742e82ed771cb7df40789882e220118045123b/cpp/src/arrow/compute/cast.cc#L101-L127

As a result, it seems these kernels may never be invoked.

### Component(s)

C++