tools: Remove pretty-print-html.

This tool has been unmaintained since our initial code
sweep to fix templates, and it has possibly bit-rotted
during unrelated code sweeps like introducing mypy, etc.

It's not documented anywhere.

The preferred method now is to run:

    ./tools/check-templates --fix

tools/pretty-print-html

@@ -1,29 +0,0 @@
-#!/usr/bin/env python3
-import os
-import sys
-from typing import List
-
-sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
-
-from tools.lib.pretty_print import pretty_print_html
-
-
-def clean_html(filenames: List[str]) -> None:
-    for fn in filenames:
-        print(f"Prettifying: {fn}")
-        with open(fn) as f:
-            html = f.read()
-        phtml = pretty_print_html(html)
-        with open(fn, "w") as f:
-            f.write(phtml)
-
-
-if __name__ == "__main__":
-    # If command arguments are provided, we only check those filenames.
-    # Otherwise, we display a message
-    filenames = sys.argv[1:]
-    if filenames:
-        clean_html(filenames)
-        sys.exit(0)
-    print("Please specify at least one file to prettify")
-    sys.exit(1)