refactor: remove unused Collection.merge()

[dilyanpalauzov]

Please check if the PR fulfills these requirements

• It's submitted to right branch according to our branching model
• It's right issue type on title
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)
• It does not introduce a breaking change or has description for the breaking change

[seonim-ryu]

Can one of the admins verify this patch?

[dongsik-yoo]

[06/01] I confirm that there is no occurrences of Collection.merge.

[jungeun-cho]

ok to test