Remove fxruby from Gemfile

Avoiding error while "rake gem:x64-mingw-ucrt"  on ruby-4.0:

```
bundle package
Source locally installed gems is ignoring #<Bundler::StubSpecification name=fxruby version=1.6.49 platform=ruby> because it is missing extensions
Bundle complete! 9 Gemfile dependencies, 10 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
Updating files in vendor/cache
  * rake-13.3.1.gem
  * fiddle-1.1.8.gem
  * mini_portile2-2.8.9.gem
Missing gem file 'fxruby-1.6.49.gem'.
rake aborted!
Command failed with status (7): [bundle package]
/home/runner/work/fxruby/fxruby/Rakefile:180:in 'block (3 levels) in <top (required)>'
/opt/hostedtoolcache/Ruby/4.0.0/x64/bin/bundle:25:in '<main>'
Tasks: TOP => gem:x64-mingw-ucrt => gem:prepare
```

Author: larskanis

Gemfile

@@ -1,7 +1,7 @@
 source 'https://rubygems.org'
 
-# Specify your gem's runtime dependencies in fxruby.gemspec
-gemspec
+# No fxruby gemspec, since "bundle package" fails on bundler-2.7+, if the extension isn't built
+# gemspec
 
 # For some reason this is required in addition to the gemspec
 # when 'bundle config force_ruby_platform true' is active: