cpp: replace usage of removed MetadataTools methods

This fixes compile errors introduced by 6b31a4ff9ac4bfc1ccf356804add10dc637366f5

The following should succeed with this PR merged, and fail without it:

```
mvn -DskipTests
cd components/scifio
mvn -DskipTests clean package cppwrap:wrap dependency:copy-dependencies
cd target/cppwrap
mkdir build
cd build
cmake ..
make
```

See also http://hudson.openmicroscopy.org.uk/view/Bio-Formats/job/BIOFORMATS-trunk-cpp/280/console and gh-505.