Remove slider package in its entirety

[n3101]

#850 (comment) identifies that slider relies on a withdrawn package and is incompatible with the recent hadoop versions to which Gaffer will migrate.

The originator @ctas582 doesn't use it any more and its removal has been announced in #967.

Remove the slider package from Gaffer 2.0 branch.

[GCHQDeveloper314]

For future reference, in case we want to reintroduce the slider module capabilities, these are some links which might be useful:

• Hadoop YARN Service - this broadly replaces Apache Slider.
• Hadoop Jira Ticket with details regarding the migration of Slider code into Hadoop
• A ticket for another project which migrated from using Slider to Hadoop YARN

[t11947]

Worth noting that the deployment scripts here currently make use of the slider work.

[t92549]

Closed by #977