DEV: Remove unused docker images (#915)

1. `monitor` uses a 9 year old base image
2. `discourse_fast_switch` uses a 6 year old base image and references ruby 2.4 and 2.5
3. `discourse_bench` uses an 8 year old base image and references postgres 9.5

Author: CvX

image/README.md

@@ -29,13 +29,3 @@ Note that the discourse user is granted "sudo" permission without asking for a p
 ### discourse_test ([discourse/discourse_test](https://hub.docker.com/r/discourse/discourse_test/))
 
 Builds on the discourse image and adds testing tools and a default testing entrypoint.
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-### discourse_bench ([discourse/discourse_bench](https://hub.docker.com/r/discourse/discourse_bench/))
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-Builds on the discourse_test image and adds benchmark testing.
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-### discourse_fast_switch ([discourse/discourse_fast_switch](https://hub.docker.com/r/discourse/discourse_fast_switch/))
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-Builds on the discourse image and adds the ability to easily switch versions of Ruby.