Remove unused output arguments.

src/@arima/estimate.m

src/@dseries/merge.m

src/modules/matlab-fame-io

src/read/load_xls_file_data.m

src/utilities/dplot/dplot.m

src/utilities/is/isdseries.m

src/utilities/misc/getlistofvariablesinscript.m

...utilities/missing/get_file_extension/get_file_extension.m

..._has_octave_forge_package/user_has_octave_forge_package.m