elf: Remove _DL_PLATFORMS_COUNT

Remove the definitions of _DL_PLATFORMS_COUNT as those are not used
anymore after removal in elf/dl-cache.c:search_cache().

Note: On x86, we can also get rid of the definitions
HWCAP_PLATFORMS_START and HWCAP_PLATFORMS_COUNT.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>