.travis.yml: Remove the unused UNRELIABLE environment variable

.travis.yml: Remove the unused UNRELIABLE environment variable

This variable was used to allow jobs to fail without spoiling the
overall result. But the required "allow_failures:" hunk has been
accidentally removed in commit 9d03f5abed ("travis.yml: Remove the
"Release tarball" job"), and it was anyway only useful while we
still had the x86 jobs here around that were our main CI jobs.
Thus let's simply remove this useless variable now.

Message-ID: <20240320104144.823425-6-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>