Remove redundant shakapacker_gem_available? checks

Since Shakapacker is a required dependency in gemspec (>= 6.0),
the shakapacker_gem_available? checks are unnecessary and misleading.

- Remove redundant checks from supports_basic_pack_generation? and supports_auto_registration?
- Remove unused shakapacker_gem_available? method entirely
- Simplify code and improve clarity
- Consistent with supports_async_loading? which correctly doesn't check availability

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

lib/react_on_rails/packer_utils.rb

@@ -45,31 +45,16 @@ def self.supports_async_loading?
     end
 
     def self.supports_basic_pack_generation?
-      return false unless shakapacker_gem_available?
-
       shakapacker_version_requirement_met?(ReactOnRails::PacksGenerator::MINIMUM_SHAKAPACKER_VERSION)
     end
 
     def self.supports_auto_registration?
-      return false unless shakapacker_gem_available?
-
       min_version = ReactOnRails::PacksGenerator::MINIMUM_SHAKAPACKER_VERSION_FOR_AUTO_REGISTRATION
       packer.config.respond_to?(:nested_entries?) && shakapacker_version_requirement_met?(min_version)
     rescue StandardError
       false
     end
 
-    def self.shakapacker_gem_available?
-      return @shakapacker_gem_available if defined?(@shakapacker_gem_available)
-
-      @shakapacker_gem_available = begin
-        require "shakapacker"
-        true
-      rescue LoadError
-        false
-      end
-    end
-
     # This returns either a URL for the webpack-dev-server, non-server bundle or
     # the hashed server bundle if using the same bundle for the client.
     # Otherwise returns a file path.