configs: Remove unneeded SYS_CONFIG_NAME from a*y17lte defconfigs

As correct default SYS_CONFIG_NAME value is now set in
board/samsung/axy17lte/Kconfig (in commit "board: samsung: Fix
SYS_CONFIG_NAME configs in axy17lte Kconfig"), the SYS_CONFIG_NAME
option can be safely removed from all a*y17lte defconfigs. That removal
doesn't change resulting .config files.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>