Remove old custom EP load infrastructure (#28095)
### Description
<!-- Describe your changes. -->
Remove load of custom EP library with arbitrary symbol name for
registration. Was used by training which has been deprecated for a
while.
AFAIK not required as any current custom EP should use the new plugin EP
infrastructure.
Originally added in https://github.com/microsoft/onnxruntime/pull/8719
### 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. -->
Resolve security issue.
