Remove mini_magick gem dependency
We can now remove mini_magick gem dependency as it is not used any more.
Further details
While !14497 (merged) originally introduced mini_magick dependency in %11.0, gitlab-org/gitlab-ce!19891 removed its dependency in %11.0.
Also the relevant documentation was introduced in !24404 (merged) in %11.8.
Edited by Takuya Noguchi