break change: remove some self-contained fields from SSL resource definition

[Guohao Wang]

hi team

Background

As we all know, APISIX supports loading specific SSL certificates through
TLS extension SNI to support https. What achieves this is APISIX's SSL
resource definition.

However, there are three fields in the SSL resource definition that can be
removed. The information read by these three words is self-contained in the
certificate itself and does not require additional storage. Additional
storage will bring additional overhead and you need to ensure that this
information is correct.

```

_M.ssl = {
    type = "object",
    properties = {...
        exptime = {
            type = "integer",
            minimum = 1588262400,  -- 2020/5/1 0:0:0
        },
...
        validity_end = timestamp_def,
        validity_start = timestamp_def,

```

Result

So I plan to remove the exptime, validity_end, validity_start fields from
the SSL resource definition.


Best Regards
guohao.wang