Remove ignore_column :issue_type from Issue model

What does this MR do and why?

Column was dropped in 16.3, so we can remove the ignore_column call in 16.4

Drop column timeline

• Column was ignored: !125320 (merged) - %16.2
• Column was dropped: !127679 (merged) - #416949 (closed) %16.3
• Remove ignore column from Issue model - This MR %16.4

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #416950 (closed)