Set *.theme files as the PHP filetype

This wasn't working with vim-polyglot enabled, as per
https://github.com/neovim/neovim/issues/23522, so I'm removing at - as
least temporarily.

after/plugin/filetype.lua

@@ -0,0 +1,5 @@
+vim.filetype.add {
+  extension = {
+    theme = "php",
+  },
+}

lib/default.nix

@@ -130,7 +130,6 @@ in rec {
       vimPlugins.nvim-spectre
       vimPlugins.vim-obsession
       vimPlugins.vim-pasta
-      vimPlugins.vim-polyglot
       vimPlugins.vim-repeat
       vimPlugins.vim-sleuth
       vimPlugins.vim-terraform