elf: EF_ARM_EABI_VERSION_UNKNOWN is no longer used, retire

EF_ARM_EABI_VERSION_UNKNOWN was used when we could run either eabi or
oabi. That was armv[45] only, and went away when we migrated to eabi
many major releases ago. No standard defines this symbol, so retire it.

Pointed out by: jrtc27
Sponsored by: Netflix