150340aeac test: remove unneeded extra_args code (josibake)
989a52e0a5 test: add extra_args to BTF class (josibake)
Pull request description:
## problem
If you try to add `extra_args` when using `TestShell`, you will get the following error:
```python
>>> import sys
>>>
>>> sys.path.insert(0, "/home/josibake/bitcoin/test/functional")
>>>
>>> from test_framework.test_shell import TestShell
>>> test = TestShell().setup(num_nodes=2, extra_args=[[],['-fallbackfee=0.0002']])
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/home/josibake/bitcoin/test/functional/test_framework/test_shell.py", line 41, in setup
raise KeyError(key + " not a valid parameter key!")
KeyError: 'extra_args not a valid parameter key!'
>>>
```
## solution
add `self.extra_args = None` so that `extra_args` is recognized as a valid parameter to be passed to `BitcoinTestFramework`
```python
>>> import sys
>>>
>>> sys.path.insert(0, "/home/josibake/bitcoin/test/functional")
>>>
>>> from test_framework.test_shell import TestShell
>>> test = TestShell().setup(num_nodes=2, extra_args=[[],['-fallbackfee=0.0002']])
2022-12-01T11:23:23.765000Z TestFramework (INFO): Initializing test directory /tmp/bitcoin_func_test_sbwthbb_
```
ACKs for top commit:
willcl-ark:
re-ACK 150340aeac
Tree-SHA512: e6fa2a780a8f2d3472c322e8cdb00ec35cb220c3b4d6ca02291eb8b41c0d8676a635fbc79c6be80e3bb71d700a2501a4b73f762478f533ae453d492d449307bb
a04121bdf9