Fix python provider, missing black formatter, dead rust-tools config

- Drop loaded_python3_provider + python3_host_prog: the guard variable
  was wrong (1 doesn't disable, 0 does) and the venv path doesn't
  survive a fresh clone. Let nvim find system python instead.
- Add "black" to Mason ensure_installed; conform already uses it for
  Python but it wasn't being auto-installed.
- Delete lua/custom/configs/rust-tools.lua: leftover from before
  rustaceanvim migration, never required anywhere.

lua/custom/configs/rust-tools.lua

@@ -1,12 +0,0 @@
-local on_attach = require("plugins.configs.lspconfig").on_attach
-local capabilities = require("plugins.configs.lspconfig").capabilities
-
-
-local options = {
-  server = {
-    on_attach = on_attach,
-    capabilities = capabilities,
-  }
-}
-
-return options

lua/custom/init.lua

@@ -1,7 +1,5 @@
 vim.g.dap_virtual_text = false
 vim.env.PATH = vim.env.PATH .. ":/Users/danesabo/.ghcup/bin"
-vim.g.loaded_python3_provider = 1
-vim.g.python3_host_prog = vim.fn.expand "~/.config/nvim/nvim_venv/bin/python"
 vim.keymap.set("i", "jj", "<ESC>")
 vim.opt.updatetime = 1000 -- 1 second delay for CursorHold events
 vim.o.sessionoptions = "blank,buffers,curdir,folds,help,tabpages,winsize,winpos,terminal,localoptions"

lua/custom/plugins.lua

@@ -21,6 +21,7 @@ local plugins = {
         "stylua",
         "clang-format",
         "fourmolu",
+        "black",
 
         --- other
         "tree-sitter-cli",