mg: fix build when host system does not have libncurses-dev

The configure script for mg checks the standard locations under /usr for
the term.h header file needed to build against ncurses; however, in the
OE cross-compile case, this is not valid, since the file will be under
the sysroot instead.  This causes the configure step to fail when the
build machine does not have the libncurses-dev package installed.

To fix the issue, remove the check.  Since mg DEPENDS on ncurses, term.h
will always be there.

Signed-off-by: Ben Shelton <ben.shelton@ni.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>