cmd_md5sum.c: remove dead code / fix warning

commit ecd72950 "Add parameter to md5sum to save the md5 sum"
adds support to build a string to be saved in the env and tries
to zero end it with str_ptr = '\0'; This does actually set the
pointer to the end of the buffer itself to zero. Since the
string was already zero terminated by the sprintf before it,
just remove the line, preventing a clang warning.

cc: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>