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: master
Request: 2.8
Request: 2.7
Request: 2.6
Patch: http://rb.dcache.org/r/6548/
Require-book: no
Require-notes: no
Acked-by: Gerd