cxl: remove dead Kconfig options

Remove the CXL_KERNEL_API and CXL_EEH Kconfig options, as they were only
needed to coordinate the merging of the cxlflash driver. Also remove the
stub implementation of cxl_perst_reloads_same_image() in cxlflash which is
only used if CXL_EEH isn't defined (i.e. never).

Suggested-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Acked-by: Ian Munsie <imunsie@au1.ibm.com>
Acked-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>