Remove erroneous javadoc invocation

When I moved to use the SBT 0.13 javadoc API instead of us invoking
javadoc directly, I accidentally left the old invocation of javadoc in
there, which always failed.

Fixes #2040
Merge pull request #2040 from jroper/2040-fix-javadoc

[error] javadoc: error - Illegal package name: "2.3-SNAPSHOT"