Decimating files in Moviemaker
You can use ffmpeg, a free command-line tool, to do this:

ffmpeg -i in.mp4 -vf "select=not(mod(n\,30)),setpts=N/FRAME_RATE/TB" -r 30 out.mp4


-vf specifies a video filterchain i.e. a sequence of filters applied to a single input.

select=not(mod(n\,30)) - this filter keeps only each 30th frame from the source.

FFmpeg works primarily within the framework of time-based containers, such as a video, which is a series of images shown in a cycle through time. The PTS is the presentation timestamp of a frame i.e. when the player should present a frame. Due to how ffmpeg works, setpts filter is needed in this case to ensure that no frames are duplicated or dropped other than what the select filter specifies. 

-r 30 is the framerate of the output. To set it to the rate of the source, you can either skip this parameter or set it manually i.e. -r 60 for a 60 fps source.