Remove CONFIG_SYS_BOOTCOUNT_SINGLEWORD

Tidy up CONFIG_SYS_BOOTCOUNT_SINGLEWORD from the whitelist as there's
no remaining uses of it left.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>

scripts/config_whitelist.txt

@@ -2194,7 +2194,6 @@ CONFIG_SYS_BOARD_VERSION
 CONFIG_SYS_BOOK3E_HV
 CONFIG_SYS_BOOTCOUNT_BE
 CONFIG_SYS_BOOTCOUNT_LE
-CONFIG_SYS_BOOTCOUNT_SINGLEWORD
 CONFIG_SYS_BOOTFILE_PREFIX
 CONFIG_SYS_BOOTMAPSZ
 CONFIG_SYS_BOOTM_LEN