Remove unnecessary REACT_ON_RAILS_SKIP_VALIDATION from generator template

The SKIP_VALIDATION environment variable is not needed in the generator
template because:

1. By the time apps use this hook, packages are already installed
2. The engine's skip_version_validation? already handles edge cases:
   - package_json_missing? check
   - running_generator? check
3. This was cargo-culted from the Ruby version which needed it for
   different reasons (loading Rails environment directly)

The shared implementation in spec/support still sets this variable
because test apps have different requirements, but user-generated
apps should rely on the engine's built-in validation logic.

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

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

Author: justin808

lib/generators/react_on_rails/templates/base/base/bin/shakapacker-precompile-hook

@@ -8,9 +8,6 @@
 
 set -e
 
-# Skip validation during precompile hook execution
-export REACT_ON_RAILS_SKIP_VALIDATION=true
-
 # Generate packs if React on Rails is configured with auto_load_bundle
 if [ -f "config/initializers/react_on_rails.rb" ]; then
   # Check for uncommented config lines (ignore lines starting with # after optional whitespace)