Duplicate keys in alpine DECLARED_TO_SPDX silently discard two license mappings

### Description

`DECLARED_TO_SPDX` in `src/packagedcode/alpine.py` has two duplicate keys. A dict literal keeps only the last value for a repeated key, so the earlier entry of each pair is dead at import time.

`'as-is'`:

- line 1619: `'as-is': 'licenseref-scancode-free-unknown',`
- line 1701: `'as-is': 'licenseref-scancode-unknown-license-reference',`

`'custom:tu-berlin-2.0'`:

- line 1690: `'custom:tu-berlin-2.0': 'licenseref-scancode-unknown-license-reference',`
- line 1715: `'custom:tu-berlin-2.0': 'tu-berlin-2.0',`

Each of the four entries carries its own comment, so the two pairs look like they were added independently without noticing the collision.

This mapping is turned straight into the substitution table applied to every Alpine package license expression:

```python
DECLARED_TO_SPDX_SUBS = {
licensing.parse(src, simple=True): licensing.parse(tgt, simple=True)
for src, tgt in DECLARED_TO_SPDX.items()
}
```

So which value wins is decided by position in the file rather than by intent. For `'as-is'` the surviving value is the less specific `licenseref-scancode-unknown-license-reference`, and the `licenseref-scancode-free-unknown` mapping never applies.

### How To Reproduce

```python
import ast, collections

src = open('src/packagedcode/alpine.py').read()
for node in ast.walk(ast.parse(src)):
if isinstance(node, ast.Dict):
counts = collections.Counter(
k.value for k in node.keys
if isinstance(k, ast.Constant) and isinstance(k.value, str)
)
for key, n in counts.items():
if n > 1:
print(f'line {node.lineno}: duplicate key {key!r} x{n}')
```

```
line 1568: duplicate key 'as-is' x2
line 1568: duplicate key 'custom:tu-berlin-2.0' x2
```

### Suggested fix

Drop the dead lines so each key appears once. Which of the two values is actually wanted for each key needs a maintainer decision, since right now it is settled by line order — I have opened a PR that removes the unreachable entries and keeps the currently-winning values, so behaviour does not change.

### System configuration

* What OS are you running on? Linux (x86_64)
* What version of scancode-toolkit was used? 33.0.0rc1, `develop` at 5ebebf2
* What installation method was used to install/run scancode? source checkout
* Python version: 3.x