change parameter --rocksdb-git-hash to readonly without being a command_line_argument

The system variable rockdb-git-hash is defined to be a command_line_argument. This needs to be changed.

select * from information_schema.system_variables where variable_name like 'rocksdb_git_hash'

VARIABLE_NAME: ROCKSDB_GIT_HASH
SESSION_VALUE: NULL
GLOBAL_VALUE: e731f4402258554812c46334dc0d9483e6cc769b
GLOBAL_VALUE_ORIGIN: COMPILE-TIME
DEFAULT_VALUE: e731f4402258554812c46334dc0d9483e6cc769b
VARIABLE_SCOPE: GLOBAL
VARIABLE_TYPE: VARCHAR
VARIABLE_COMMENT: Git revision of the RocksDB library used by MyRocks
NUMERIC_MIN_VALUE: NULL
NUMERIC_MAX_VALUE: NULL
NUMERIC_BLOCK_SIZE: NULL
ENUM_VALUE_LIST: NULL
READ_ONLY: YES
COMMAND_LINE_ARGUMENT: REQUIRED
GLOBAL_VALUE_PATH: NULL
1 row in set (0.00 sec)

COMMAND_LINE_ARGUMENT needs to be NULL
DEFAULT_VALUE needs to be NULL