mysql-compatibility: update ZERO_DATE docs (#4436)

Co-authored-by: TomShawn <41534398+TomShawn@users.noreply.github.com>

mysql-compatibility.md

@@ -172,13 +172,6 @@ TiDB supports most [SQL modes](/sql-mode.md):
 + TiDB uses all time zone rules currently installed in the system for calculation (usually the `tzdata` package). You can use all time zone names without importing the time zone table data. You cannot modify the calculation rules by importing the time zone table data.
 + MySQL uses the local time zone by default and relies on the current time zone rules built into the system (such as when to start daylight saving time) for calculation; and the time zone cannot be specified by the time zone name without [importing the time zone table data](https://dev.mysql.com/doc/refman/5.7/en/time-zone-support.html#time-zone-installation).
 
-#### Zero month and zero day
-
-By default, the `NO_ZERO_DATE` and `NO_ZERO_IN_DATE` modes are enabled in TiDB, which is the same in MySQL, but TiDB and MySQL handle the two SQL modes differently in the following aspects:
-
-- The two SQL modes above are enabled in TiDB in the non-strict mode where no warning is prompted for inserting values of zero month/zero day/zero date. In MySQL, such `INSERT` operations prompt warning.
-- In the strict mode, when `NO_ZERO_DATE` is enabled, you can still insert values of zero date; when `NO_ZERO_IN_DATE` is enabled, you cannot insert date of zero month/zero day. In the strict mode of MySQL, you can insert neither of them.
-
 ### Type system differences
 
 The following column types are supported by MySQL, but **NOT** by TiDB: