[ARMNN EP] Remove ArmNN Execution Provider (#27447)
### Description
Remove the ArmNN Execution Provider: build flags, provider registration,
APIs, tests, and
packaging references are deleted, along with the ArmNN provider
implementation sources.
### Motivation and Context
ArmNN EP has been deprecated/removed per release notes, and the repo
should no longer expose
build options, APIs, or documentation for it. This change aligns the
codebase with that
removal.
