Remove the default for build parameter SCALA_VER_SUFFIX

As the empty string is a a valid value.

I checked if this variable is used in other builds in the same way.
It is only used in this job:

    % ack SCALA_VER_SUFFIX
    job/scala-release-2.11.x
    30:# but, no default for SCALA_VER_SUFFIX, "" is a valid value for the final release.
    80:SCALA_VER="$SCALA_VER_BASE$SCALA_VER_SUFFIX"
    272:    -Dmaven.version.suffix=$SCALA_VER_SUFFIX\
    321:#     -Dmaven.version.suffix=$SCALA_VER_SUFFIX\
Merge pull request #96 from retronym/topic/scala-ver-suf

Remove the default for build parameter SCALA_VER_SUFFIX