config/lnl.toml: drop now invalid signed_pkg.partition_usage

Fixes commit fbea593 ("rimage: mtl: fix key slot setup based on imr
type") which broke LNL compilation like this:

```
error: 1 unparsed keys left in 'signed_pkg'
error: key 'signed_pkg' parsing error
```

Of course LNL is never going to be released on this stable-v2.7 branch
but this one-line fix is just the smallest and simplest fix to keep
everything building and green. It also keeps everything simple and
consistent.

This commit does not change the final image (compared to before rimage
commit fbea593) because the default value is now 0x20 + IMR type.

Fixes are normally submitted to the main branch first and then cherry
picked to stable branches. However fbea593 has been made in
stable-v2.7 first so make this change also in stable-v2.7 first for
consistency.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

config/lnl.toml

@@ -44,7 +44,6 @@ length = "0x0"  # calculated by rimage
 
 [signed_pkg]
 name = "ADSP"
-partition_usage = "0x23"
 [[signed_pkg.module]]
 name = "ADSP.met"