Remove Kernel's define_singleton_method patch

[hmnhf]

• This PR addresses this comment.
• The patch was added in commit 23e8e18 back in 2012 to support define_singleton_method for Ruby 1.8.
• Since Ruby 1.8's EOL was in 2014, and KAUI requires Ruby 3.2.2+, it should be safe to remove the patch.
• I made sure all tests pass after the removal.
  • define_singleton_method is only used in TagDefinition to define TagDefinition.all_for_* methods.
  • The defined methods are used in places like account_tags#edit which have tests.

[pierre]

Thank you!

To give additional colour, this was required to support legacy Rails applications, as we used to only provide the kaui gem as a mountable engine. Nowadays, users should instead consume the standalone Kaui application, which supports various forms of customizations.