Use Doctrine Platform to generate SQL query for table truncation.

Possible replacement for #10720 (and as a result hopefully fixes #10708).

In constrast to #10720 this does not introduce further DB switching, but instead removes DB switching completely.

The new expected behaviour is as follows (also see https://github.com/owncloud/core/pull/10720#issuecomment-53894567):
MySQL uses TRUNCATE (as before)
Sqlite uses DELETE FROM (as before)
MSSQL uses TRUNCATE TABLE (as in #10720)
Oracle uses TRUNCATE TABLE (previously used DELETE FROM, so this needs testing)

I have not done any testing, I will leave that to people with LDAP setup or unit tests.

@blizzz @DeepDiver1975 @butonic @gig13