dcache-core, alarms: remove postgresql precondition on schema

There is nothing in the liquibase schema for alarms which specifically requires postgresql.
Removing this precondition allows the rdbms option to be tested in system-test using hsqldb.

Target: 2.7
Patch: http://rb.dcache.org/r/6548/
Require-book: no
Require-notes: no
Acked-by: Gerd
Committed: b84b3fa2fe66b44a9f24dcdeda8cc344780f492b
Merge pull request #403 from alrossi/fix/2.7/alarms-schema

dcache-core, alarms: remove postgresql precondition on schema