[JITLink] Some cleanups to EHFrameSupport (#66707)

 * Remove unused variable.
 * Error on existing edge at CIE pointer field.
 * Simplify CFI processing in `EHFrameEdgeFixer`: The code expects
`DWARFRecordSectionSplitter` to split each CFI record into its own
block, so remove loop over possibly multiple entries in one block.