- Remove excess build flag from debian/rules that does nothing, introduced
  wrongly in commit 7cbde2d0. Instead
  implement the embedded server build skipping on Travis-CI correctly.
- Simplify structure by doing all Travis-CI slimdown in one step.
- Remove unnecessary -e from sed, as it does nothing. When regex is needed,
  use -r. Move -i to last so it is close to the file name it has an
  argument.
- Remove backwards compat checks that are no longer relevant as neither
  Debian Jessie (was before Stretch) nor Ubuntu Trusty (before Xenial)
  are supported nor built for anymore. For example the GCC 4.8 check
  if not relevant anymore, since Debian Jessie already has 4.9 and
  Ubuntu Xenial has 5.3 and there is no GCC < 4.8 around anymore.
- Skip building ColumnStore on both Travis-CI and Gitlab-CI as it is
  way too slow (time) and big (disk space) to pass.