Remove manag_.groovy

This was an experiment that was not adopted.

Retaining just creates confusion.

For reference, these are the commits related to the file:

o    68d4d22f kk [JENKINS-8446] it's better to let those directories setting be a part of JENKINS_HOME rather than being independent configuration properties, since they aren't orthogonal.
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: o  82917200 kk making it more type-safe
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: o  8c7c1c62 kk this file is now auto-generated
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o  61768cc0 kk not specifying the file extension so that different pieces can be implemented by different view technologies
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o  12ad3b02 kk replaced i18n by '_' since it shows up a lot.
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o  00bc3298 kk experimental re-write of the "manage Jenkins" view by Groovy
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