remove sourcemapping from the input file

I'm using [gulp-concat-sourcemap](https://github.com/mikach/gulp-concat-sourcemap) and I'm having an issue with inline sourcemap.
TLDR: The output files contain the original sourcemaps. 
For example 

    gulp.src(['1.css', '2.css', '3.css'])
       .pipe(sourceMaps.init())
          .pipe(concat('output.css'))
       .pipe(sourceMaps.write('map'))
       .pipe(gulp.dest('output-dir'))

In this case the `output.css` will contain the sourcemapping from the `1.css`, `2.css`, `3.css` and the mapping file (generated by `sourceMaps.write('map')`) will contain also the `1.css`, `2.css`, `3.css` sourcemapping.