rdbms: Remove weird use of serialize() in MssqlBlob and DatabaseMssql

rdbms: Remove weird use of serialize() in MssqlBlob and DatabaseMssql

The use in MssqlBlob could never be reached, since the test used `&&`
rather than `||`.

The use in DatabaseMssql should never be reached, and all our other
Database subclasses will give PHP warnings or errors when it tries to
stringify an array or object rather than silently serializing. MSSQL
should be consistent there. Further, MSSQL never *un*serializes, so it
seems rather useless anyway.

Change-Id: I7b193c6be28662edb38d416ac02371f7a293ab56