Delete ISSUABLE_TYPE and ISSUABLE_INDEX object constants

What does this MR do and why?

This MR replaces ISSUABLE_TYPE usages to match the TYPE_ISSUE and TYPE_MERGE_REQUEST constants for consistency since TYPE_* constants are the main set of constants for issuables.

Also replaces the ISSUABLE_INDEX object with a string since there's only one single usage of it.

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