Remove `mg979/tabline.nvim`

[ingenarel]

Because the original repo doesn't exist now.

Checklist:

• [ ] The plugin is specifically built for Neovim, or if it's a colorscheme, it supports treesitter syntax.
• [ ] The lines end with a .. This is to conform to awesome-list linting and requirements.
• [ ] The title of the pull request is Add/Update/Remove `username/repo` (notice the backticks around `username/repo`) when adding a new plugin.
• [ ] The description doesn't mention that it's a Neovim plugin, it's obvious from the rest of the document. No mentions of the word plugin unless it's related to something else. No .. for Neovim.
• [ ] The description doesn't contain emojis.
• [ ] Neovim is spelled as Neovim (not nvim, NeoVim or neovim), Vim is spelled as Vim (capitalized), Lua is spelled as Lua (capitalized), Tree-sitter is spelled as Tree-sitter.
• [ ] Acronyms should be fully capitalized, for example LSP, TS, YAML, etc.

[tversteeg]

Thanks for the PR! Fixes #1258.