Remove ZpoolMonitor and ZPoolExpandNotice

[basil]

This plugin contains an administrative monitor to check if the Jenkins home is running in EC2 on ZFS and report a message if the zpool is running out of space. It is unclear why this functionality lives in the EC2 plugin, as it is not cloud-specific. Moreover, it is probably used by very, very few people, since the relevant libzfs integration hasn't been updated in 7 years, during which I imagine most people running illumos or Solaris on EC2 have since moved to Linux. If anyone truly misses this functionality, it could be moved to Jenkins core and adapted to run regular zfs or zpool commands rather than using a complex Java/C binding, but in the meantime it seems fine to just delete this code and slim down the JPI file of the EC2 plugin to avoid an unnecessary library.

Testing done

mvn clean verify -Dtest=InjectedTest

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