upload-plugin-from-maven: work around an imagej-updater bug

The imagej-update version 0.3.0 unfortunately has a bug: when removing
an update site via the command line, it is really removed (rather than
simply deactivated) and the files that are still associated with said
update site cause a NullPointerException.

The bug has been fixed in the meantime, but we can also work around the
issue (while avoiding one checksum run, i.e. while speeding things up,
too).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Port the imagej-updater work-around to upload-update-site.sh

Same story as with upload-plugin-from-maven.sh...

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