HHH-9500: H2Dialect shoud not drop constraints

The constraints are automatically dropped when the table is also dropped.
Trying to drop a constraint on a table that doesn't exist present unnecessary error messages.

(cherry picked from commit 7854bc6769c1f0b73a6b54216fd1586fc19feffa)
HHH-6670: H2Dialect should use "if exists" when dropping sequences

(cherry picked from commit 60d4fe5001cdc31affe800c73f32f13ac274c8b8)