Remove protobuf install in travis build

[asfimport]

Now that we use maven protoc plugin (PARQUET-1280), we do not need any more to build protoc ourselves. So we could remove protobuf build at the beginning of the travis build.

Reporter: Qinghui Xu / @qinghui-xu
Assignee: Qinghui Xu / @qinghui-xu

Related issues:

• [parquet-protobuf] Use maven protoc plugin (requires)

PRs and other links:

• GitHub Pull Request #592

_{Note: This issue was originally created as PARQUET-1492. Please see the migration documentation for further details.}