Remove orphaned snowflake_flatten_validation.sql script (#22938)

## Which issue does this PR close?

- Closes #.

## Rationale for this change

This file seems to be a leftover of #21593. I see what its trying to do,
but most developers don't have access to a Snowflake instance, and the
transformations are already all coded in tests.

I looked into turning it into some sort of SLT tests, but I couldn't
find a Snowflake-provided mock server.

## What changes are included in this PR?

- Remove `snowflake_flatten_validation.sql`

## Are these changes tested?

Existing tests. I ran all the tests in `datafusion-sql` to make sure
there's no hidden dependency I missed.

## Are there any user-facing changes?

None

Signed-off-by: Adam Gutglick <adamgsal@gmail.com>

Author: AdamGS