libnewt-python: remove make var LIBNEWTSH

libnewt-python: remove make var LIBNEWTSH

Var LIBNEWTSH which points to libnewt.so in STAGING_LIBDIR is passed to
make. But during do_compile, LIBNEWTSH is rebuilt. Check the log that
gcc populates file into STAGING_LIBDIR directly:

i586-poky-linux-gcc  -m32 -march=i586
--sysroot=/poky/builds/build/tmp/sysroots/qemux86 -shared -o
/poky/builds/build/tmp/sysroots/qemux86/usr/lib/libnewt.so.0.52.18
...

It is not a proper operation export file into STAGING_LIBDIR during
compile. So remove the var LIBNEWTSH.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>