BUG: using df.drop(labels=x, level='level_name') does not update df.index.levels
- [x] I have checked that this issue has not already been reported.

- [ ] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
arrays = [['bar', 'bar', 'baz', 'baz', 'foo', 'foo', 'qux', 'qux'], ['one', 'two', 'one', 'two', 'one', 'two', 'one', 'two']]
tuples = list(zip(*arrays))
index = pd.MultiIndex.from_tuples(tuples, names=['first', 'second'])
s = pd.Series(np.random.randn(8), index=index)
s2 = s.drop(labels='one', level='second') #drop the label 'one' from the second level
print(s2.index.levels) #the levels still include the label 'one' in the second level
```
outputs:
`FrozenList([['bar', 'baz', 'foo', 'qux'], ['one', 'two']])`
despite dropping `one` from the second level

#### Problem description

When calling `df.drop(labels='one', level='second')`, one would expect that the dropped label would no longer appear in `df.index.levels` since all rows with that label are dropped. However, the label persists in the `df.index.levels` despite not showing up in `df.index.get_level_values('second')`. I would expect that `df.index.levels` only include labels of entries currently in `df`, not ones that were previously there but dropped.

#### Expected Output
`FrozenList([['bar', 'baz', 'foo', 'qux'], ['two']])`
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : d9fff2792bf16178d4e450fe7384244e50635733
python           : 3.8.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.14.182-llgrid-10ms
Version          : #1 SMP Fri May 29 11:57:47 EDT 2020
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : C
LANG             : C.UTF-8
LOCALE           : None.None

pandas           : 1.1.0
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.2
setuptools       : 49.3.1.post20200810
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.17.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 1.0.0
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
