Travis-CI removal

Due a Travis-CI resources policy change (migration from travis-ci.org to travis-ci.com and related changes in available resources), migration to Eclipse-CI Jenkins environment started on 30-Jun-2021.
This is replacement of Travis-CI environment used to verify pull requests.
See previous PRs:
eclipse-ee4j#1195
eclipse-ee4j#1202
eclipse-ee4j#1203
eclipse-ee4j#1205
This final change removes .travis.yml file as this file is not used anymore and Eclipse-CI Jenkins environment is active now see:
https://ci.eclipse.org/eclipselink/job/eclipselink-pull-request-verifier/
https://ci.eclipse.org/eclipselink/job/eclipselink-pull-request-verifier/view/change-requests/

Signed-off-by: Radek Felcman <radek.felcman@oracle.com>