remove old syntax for extended statistics

[eurekaka]

Feature Request

Is your feature request related to a problem? Please describe:

Not a problem, just a remained work for #1138.

Describe the feature you'd like:

We support ALTER TABLE ... ADD STATS_EXTENDED now, so we can remove the old CREATE STATISTICS syntax.

Describe alternatives you've considered:

N/A
Teachability, Documentation, Adoption, Migration Strategy:

N/A