admin/JM-release.sh: Do not specify commands with abspath

admin/JM-release.sh: Do not specify commands with abspath

Previously sed and rm commands are specified using abspath,
but abspath of these commands may vary depending on platforms.
At least "sed" is located at /usr/bin/sed in MacOSX.
JM-release.sh is just used to copy draft manpages to release
directory and it would be nice if the script works on more platforms.