Removed enum member UNKNOWN_PROP causing building failurs on thrift.

[xtcyclist]

RT.

enum NullType {
    __NULL__ = 0,
    NaN      = 1,
    BAD_DATA = 2,
    BAD_TYPE = 3,
    ERR_OVERFLOW = 4,
    UNKNOWN_PROP = 5,
    DIV_BY_ZERO = 6,
    OUT_OF_RANGE = 7,
} (cpp.enum_strict, cpp.type = "nebula::NullType")

When UNKNOWN_PROP = 5 was removed, generated thrift codes may not be compiled in some cases.