x86: Remove HWCAP_START and HWCAP_COUNT

Both defines are not used anymore.  Those were only used for
_dl_string_hwcap(), which itself was removed with commit
ab40f203
"elf: Remove _dl_string_hwcap"

Just clean up.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>