Fix!: cast less aggressively

[georgesittas]

This addresses the following issue (note the additional TIMESTAMP added):

>>> import sqlglot
>>> sql = "SELECT DATE_FORMAT(CAST(FROM_UTC_TIMESTAMP(foo::TIMESTAMP, 'America/Los_Angeles') AS TIMESTAMP), 'yyyy-MM-dd HH:mm:ss') AS foo FROM t"
>>> expr.sql("databricks")
"SELECT DATE_FORMAT(CAST(CAST(FROM_UTC_TIMESTAMP(CAST(foo AS TIMESTAMP), 'America/Los_Angeles') AS TIMESTAMP) AS TIMESTAMP), 'yyyy-MM-dd HH:mm:ss') AS foo FROM t"

[georgesittas]

Not sure if we want to do this at the exp.cast level instead of manually using cast_unless everywhere, wdyt @tobymao? I.e. replace cast with cast_unless.

[tobymao]

let's change the behavior of cast to be cast unless, and then allow for flags to 1. change the type, 2. allow users to add cast no matter what, so this is a breaking change to cast

let's delete cast unless

[tobymao]

this is a breaking change