Revert "Delete PostgresI2b2HelperService..."

This reverts commit b393cfc1974c8f03dd8d47eaeb91ef6959086f0b.

This could also be resolved by simply replacing the implementation of
the bean i2b2HelperService with the class I2b2HelperService, as
I2b2HelperService and PostgresI2b2HelperService have the same content.
But there are other Postgres specific classes that are identical copies
of their original files and they should probably be handled together, if
not by deleting the postgres specific versions, at least by extracting a
common interface.