Clarify when protobuf dependency builds protoc (#20542)
### Description
<!-- Describe your changes. -->
Currently figuring out if the protobuf dependency is building protoc it
is a little obtuse and inconsistent
* in some places we directly set protobuf_BUILD_PROTOC_BINARIES to OFF
to indicate the protobuf dependency is not building protoc
* e.g. macOS/iOS/visionOS builds
* for a user provided protoc path we don't set
protobuf_BUILD_PROTOC_BINARIES, and inside protobuf_function.cmake that
determines if `protobuf::protoc` is added as a dependency or not
*
https://github.com/microsoft/onnxruntime/blob/0dda8b0c449009dee5e9cdf3ccaa4fbbacf04dd0/cmake/external/protobuf_function.cmake#L40-L45
To be more consistent/explicit, set protobuf_BUILD_PROTOC_BINARIES to
OFF when ONNX_CUSTOM_PROTOC_EXECUTABLE set and valid.
Remove outdated script that built and external protoc binary which was
used in later builds. The build setup will fetch a pre-built protoc so
there's no need for this additional build.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Make it easier to figure out if protoc is coming from the protobuf
dependency.
