Remove legacy Guardfile (#201)

This Guardfile appears to have been introduced as part of a personal
development setup, using guard-rspec to automatically run tests and send
notifications to iTerm2 on macOS.

It was added 8 years ago and is likely no longer in active use. There
are no dependencies on guard or guard-rspec in the Gemfile, and no
references to Guard elsewhere in the project.

Additionally, the file is a symbolic link, which is not supported on
Windows and is excluded via the gemspec.

As this project transitions from a personal project to a team-based
development workflow, this PR proposes removing the file to simplify the
repository and improve cross-platform compatibility.

If Guard is still intended to be used, feel free to close this PR.

Thank you for your consideration.

Author: kojix2

Guardfile

@@ -22,8 +22,7 @@ Gem::Specification.new do |spec|
 
   spec.files         = `git ls-files -z`.split("\x0").reject do |f|
     case f
-    when %r{^Guardfile},  # NOTE: Skip symlink for Windows
-         %r{^ext/pycall/spec_helper/},
+    when %r{^ext/pycall/spec_helper/},
          %r{^(test|spec|features)/}
       true
     else