Fixed a bug in IsoMovieBox.remove_sample_before/after_point() and MovieBox-bound range in fragmented source

I'd erroneously added short circuits to IsoMovieBox
 .remove_sample_before/after_point() when AccessPoint indicated is_at_start/
 is_at_end. This didn't account for operating on the non-primary track.
 The short circuits are in MediaBox.remove_sample_refs_before/after().

Also added missing logic to remove all fragments in a fragmented movie when
 the range start and end are bound to the MovieBox-described samples.

Tested inter MovieBox ranges with B-AVC_MP4_MP_SD-01.1_1.MP4 and verified
 resulting mp4.