remove use of use-deprecated-sizes

What does this MR do?

Remove use-deprecated-sizes from the file app/assets/javascripts/ide/components/merge_requests/list.vue. New icon sizes are now 16 instead of 18 since it is the closest allowed value.

Screenshots (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

Conformity

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Related to #324779 (closed)