Cleanup/simplify install from source instructions
Cleanup/simplify install from source instructions

The install instruction require installing several packages which are no longer needed now that we only support Py3.6 and up. Hence, doing some cleanup.

Before installing the pip package, but after a successful build on a clean Python 3.8 environment:

```console
(venv) mihaimaruseac@ankh:/tmp/tf/cp/tf$ pip list
Package             Version
------------------- -------
Keras-Preprocessing 1.1.2
numpy               1.19.2
pip                 20.2.3
pkg-resources       0.0.0
setuptools          44.0.0
six                 1.15.0
wheel               0.35.1
```

After installing:
```
(venv) mihaimaruseac@ankh:/tmp/tf/cp/tf$ pip list
Package                Version
---------------------- ---------
absl-py                0.10.0
astunparse             1.6.3
cachetools             4.1.1
certifi                2020.6.20
chardet                3.0.4
flatbuffers            1.12
gast                   0.4.0
google-auth            1.22.1
google-auth-oauthlib   0.4.1
google-pasta           0.2.0
grpcio                 1.32.0
h5py                   2.10.0
idna                   2.10
Keras-Preprocessing    1.1.2
Markdown               3.3.1
numpy                  1.19.2
oauthlib               3.1.0
opt-einsum             3.3.0
pip                    20.2.3
pkg-resources          0.0.0
protobuf               3.13.0
pyasn1                 0.4.8
pyasn1-modules         0.2.8
requests               2.24.0
requests-oauthlib      1.3.0
rsa                    4.6
setuptools             44.0.0
six                    1.15.0
tensorboard            2.3.0
tensorboard-plugin-wit 1.7.0
tensorflow             2.4.0
tensorflow-estimator   2.3.0
termcolor              1.1.0
typing-extensions      3.7.4.3
urllib3                1.25.10
Werkzeug               1.0.1
wheel                  0.35.1
wrapt                  1.12.1
```
tensorflow/docs,tensorflow/docs,tensorflow/docs