Remove custom issue template

Request has been updated so this irrelevant

.github/ISSUE_TEMPLATE/Custom.md

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-name: 'Security: Request/Hawk/Hoek is being flagged'
-about: We cannot address this till we drop support for Node < 6. Subscribe to 2355
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-See https://github.com/sass/node-sass/issues/2355 for the reason we can't upgrade the Request version that your security scanner is flagging.