[build] Remove set_sources_assignment_filter

This feature was removed from upstream GN.

Bug: 61489
Change-Id: I71910cf21439f4b457049ee57626a553b4520683
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/flatbuffers/+/492298
Reviewed-by: Aaron Wood <aaronwood@google.com>
1 file changed