bump-versions-in-pom: remove special handling of scijava-maven-plugin

Somewhere along the lines, we switched away from specifying the groupId
via properties, therefore no special handling is required any longer.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
bump-versions-in-pom: special-case some artifactIds

We had a convention to use the full artifactId in the properties
defining the current versions, but at some stage that got nixed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
bump-versions-in-pom: do not replace versions with empty strings, ever

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
bump-versions-in-pom: work around BSD sed's lack of \| handling

Too bad: GNU sed and BSD sed differ in various ways, all of them
annoying.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>