perf: remove unnecessary multidex library

Android 5 and up supports multiple dex files natively, there is no need to include the library when minSdkVersion is 21. https://developer.android.com/build/multidex#mdex-on-l

[ ] Code formatting and import sorting has been done with dart format lib/ test/ and dart run import_sorter:main --no-comments
[x] The commit message uses the format of Conventional Commits
[ ] The commit message describes what has been changed, why it has been changed and how it has been changed
[ ] Every new feature or change of the design/GUI is linked to an approved design proposal in an issue
[ ] Every new feature in the app or the build system has a strategy how this will be tested and maintained from now on for every release, e.g. a volunteer who takes over maintainership

krille-chan / fluffychat