Remove unused custom target JSON files

8521530f4938 ("Fix __divsi3 and __udivsi3 on thumbv6m targets") removed
tests that use these `thumb*-linux` target files in favor of tests that
use the `thumb*-none` targets, which are available via Rustup. The JSON
files haven't been used since then and are outdated, so remove them.

Author: tgross35