Remove mini.splitjoin

I already have `treesj` which works better for PHP and Nix compared to
the `mini.splitjoin`.

after/plugin/treesj.lua

@@ -0,0 +1,10 @@
+local tsj = require "treesj"
+
+tsj.setup {
+  use_default_keymaps = false,
+}
+
+local nmap = require "opdavies.keymap".nmap
+
+nmap { "gJ", tsj.join }
+nmap { "gS", tsj.split }