tools: Remove find-add-class tool.

I added this tool a few years ago, and I did have
a vision for how it would improve our codebase, but
I can't remember exactly where I was going with it.

At this point the tool is just a little too noisy
to be helpful.  An example of it creating confusion
was a recent PR where somebody was patching
user_circle_class in the PM list, and we already
had similar code in the buddy list, because they
use the same CSS.  I mean, there was possibly a way
that the code could have been structured to remove
some of the duplication, but it probably would have
just moved the complexity around.

I just don't think it's worth maintaining the tool
at this point.

tools/lint

@@ -51,8 +51,6 @@ def run():
         'frontend': ['js', 'ts', 'css', 'scss', 'hbs', 'html', 'lock'],
     }, exclude=EXCLUDED_FILES)
 
-    linter_config.external_linter('add_class', ['tools/find-add-class'], ['js'],
-                                  description="Compares addClass() between JavaSsript and CSS.")
     linter_config.external_linter('css', ['node', 'node_modules/.bin/stylelint'], ['css', 'scss'],
                                   fix_arg='--fix',
                                   description="Standard CSS style and formatting linter "