.github: workflow: removed legacy RTOS platforms from Zephyr build

Legacy platforms not meant to work with Zephyr were added
in commit ".github: compile-test multiple zephyr revisions + IPC4"
(8543f5c).
Removed them from CI as they never should be built in the first place.

Signed-off-by: Andrey Borisovich <andrey.borisovich@intel.com>

Author: Andrey Borisovich

.github/workflows/zephyr.yml

@@ -18,9 +18,6 @@ jobs:
         # many lines. Pay attention to COMMAS.
         IPC_platforms: [
           # - IPC3 default
-          apl cnl,
-          icl jsl,
-          tgl tgl-h,
           imx8 imx8x imx8m,
           # - IPC4 default
           mtl,