Drop unused OS column from the drivers list

[dennwc]

OS column is currently printed in the driver list:

$ bblfshctl driver list
+------------+------------------------------------------+---------+--------+---------+--------+-------------+------------------------------+
|  LANGUAGE  |                  IMAGE                   | VERSION | STATUS | CREATED |   OS   |     GO      |            NATIVE            |
+------------+------------------------------------------+---------+--------+---------+--------+-------------+------------------------------+
| python     | docker://bblfsh/python-driver:latest     | v2.8.2  | beta   | 2 days  | alpine | 1.10-alpine | python:3.6-alpine            |
| csharp     | docker://bblfsh/csharp-driver:latest     | v1.4.1  | beta   | 6 days  |        | 1.10        | microsoft/dotnet:2.1-runtime |

But since we changed the build system this column has no effect on the driver and is probably outdated.

We should drop this column from the manifest and the driver list output.