ci: don't build for both thumbv6 and thumbv7.

We have 16 jobs now, starts to hit the GHA free limit of 20. There's no
advantage in testing thumbv6 and thumbv7 because they're almost identical
and nothing in e-h depends on them, so remove thumbv6.

Author: Dirbaio

.github/workflows/test.yml

@@ -21,7 +21,6 @@ jobs:
           - nightly
         target:
           - x86_64-unknown-linux-gnu
-          - thumbv6m-none-eabi
           - thumbv7m-none-eabi
         features:
           - ''