Deprecate groupby() squeeze option
#### Code Sample

```python
import pandas as pd
print(pd.DataFrame([{
    'A': 1,
    'B': 1,
}, {
    'A': 2,
    'B': 2,
}, {
    'A': 2,
    'B': 3,
}]).groupby('A', squeeze=True).count())

```
#### Problem description

I expected `.groupby(squeeze=True)` to, well, squeeze, and `count()` to return a Series. Instead `squeeze=True` doesn't seem to do anything, and `count()` returns a DataFrame.

A workaround is to write `.groupby('A').count().squeeze()`, which does work.

#### Expected Output

```
A
1    1
2    2
Name: B, dtype: int64
```

#### Actual output

```
   B
A
1  1
2  2
```

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.0-4-amd64
machine          : x86_64
processor        :
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 1.0.1
numpy            : 1.17.4
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 18.1
setuptools       : 44.0.0
Cython           : None
pytest           : 4.6.9
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.0
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 4.6.9
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None
</details>
