Fix warning of vuepress plugins have been used multiple times

[hustcer]

Fix these warnings while runing npm run build, they are already included in the default theme:

warning plugin @vuepress/plugin-git has been used multiple times, only the last one will take effect
warning plugin @vuepress/plugin-back-to-top has been used multiple times, only the last one will take effect
warning plugin @vuepress/plugin-medium-zoom has been used multiple times, only the last one will take effect
warning plugin @vuepress/plugin-copy-code has been used multiple times, only the last one will take effect