Revert "check-release.pl: assume that maven-helper.sh's executable bit is unset"

Whoops. This only works if maven-helper.sh is in the *current* directory.

This reverts commit 1a92753e4bc1f4d247a685850511b5ff07af0b9f.

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