[CI] Remove Travis related files and disable Travis

[haroldrandom]

Fix #1184

Travis trigger other test other than the modified code:

• Add support for pg v2 #1318
• remove azext.maxCliCoreVersion #1322

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This checklist is used to make sure that common guidelines for a pull request are followed.

General Guidelines

• Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• Have you run python scripts/ci/test_index.py -q locally?

For new extensions:

• My extension description/summary conforms to the Extension Summary Guidelines.

[azuresdkci]

If this PR is for a new extension or change to an existing extension, use the following to try out the changes in this PR:

docker run -it microsoft/azure-cli:latest
export EXT=<NAME>
pip install --upgrade --target ~/.azure/cliextensions/$EXT "git+https://github.com/haroldrandom/azure-cli-extensions.git@remove-travis#subdirectory=src/$EXT&egg=$EXT"