Don't use an older JRuby with oraclelinux-7 (#16499)

A recent PR (elastic/ci-agent-images/pull/932) modernized the VM images
and removed JRuby 9.4.5.0 and some older versions.

This ended up breaking exhaustive test on Oracle Linux 7 that hard coded
JRuby 9.4.5.0.

PR #16489 worked around the
problem by pinning to the new JRuby, but actually we don't
need the conditional anymore since the original issue
jruby/jruby#7579 (comment) has
been resolved and none of our releasable branches (apart from 7.17 which
uses `9.2.20.1`) specify `9.3.x.y` in `/.ruby-version`.

Therefore, this commit removes conditional setting of JRuby for
OracleLinux 7 agents in exhaustive tests (and relies on whatever
`/.ruby-version` defines).

Author: dliappis

.buildkite/scripts/exhaustive-tests/generate-steps.py

@@ -147,11 +147,6 @@ def acceptance_linux_steps() -> list[typing.Any]:
 set -eo pipefail
 source .buildkite/scripts/common/vm-agent-multi-jdk.sh
 source /etc/os-release
-if [[ "$$(echo $$ID_LIKE | tr '[:upper:]' '[:lower:]')" =~ (rhel|fedora) && "$${VERSION_ID%.*}" -le 7 ]]; then
-  # jruby-9.3.10.0 unavailable on centos-7 / oel-7, see https://github.com/jruby/jruby/issues/7579#issuecomment-1425885324 / https://github.com/jruby/jruby/issues/7695
-  # we only need a working jruby to run the acceptance test framework -- the packages have been prebuilt in a previous stage
-  rbenv local jruby-9.4.8.0
-fi
 ci/acceptance_tests.sh"""),
         }
         steps.append(step)