Remove ad-hoc dependency from Gemfile

This code was introduced in the following.
367e428

However, ember-cli-rails-assets 0.7.0 solved Rails ~>7.0 issues.

Gemfile

@@ -11,7 +11,6 @@ else
 end
 
 gem "rails", rails_constraint
-gem "ember-cli-rails-assets", github: "seanpdoyle/ember-cli-rails-assets" if rails_version == "main" || Gem::Version.new(rails_version) >= Gem::Version.new("7.0")
 gem "high_voltage", "~> 3.0.0"
 gem "webdrivers", "~> 5.0"
 gem "webrick"