ci: manifest: Drop west label for manifest updates

This commit updates the manifest workflow to not label the pull
requests updating the west manifest (`west.yml`) with the `west` label
because the `manifest` label alone is enough for this purpose and the
`west` label can be easily confused with the `area: West` label.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

.github/workflows/manifest.yml

@@ -23,5 +23,5 @@ jobs:
           checkout-path: 'zephyrproject/zephyr'
           label-prefix: 'manifest-'
           verbosity-level: '1'
-          labels: 'manifest, west'
+          labels: 'manifest'
           dnm-labels: 'DNM'