selftests/sync: Remove the deprecated config SYNC

SYNC was removed since
aff9da10 ("staging/android: make sync_timeline internal to sw_sync")
LKP/0Day will check if all configs listing under selftests are able to
be enabled properly.

For the missing configs, it will report something like:
LKP WARN miss config CONFIG_SYNC= of sync/config

- it's not reasonable to keep the deprecated configs.
- configs under kselftests are recommended by corresponding tests.
So if some configs are missing, it will impact the testing results

CC: Philip Li <philip.li@intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/sync/config

 CONFIG_STAGING=y
 CONFIG_ANDROID=y
-CONFIG_SYNC=y
 CONFIG_SW_SYNC=y