ci: drop legacy platforms from firmware build

We just care about QEMU on these platforms.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

.github/workflows/build-firmware.yml

@@ -27,9 +27,6 @@ jobs:
     strategy:
       matrix:
         board:
-          - snowy_bb2
-          - spalding_bb2
-          - silk_bb2
           - asterix
           - obelix_dvt
           - obelix_pvt