DEV: Fix various rubocop lints (#24749)

These (21 + 3 from previous PRs) are soon to be enabled in rubocop-discourse:

Capybara/VisibilityMatcher
Lint/DeprecatedOpenSSLConstant
Lint/DisjunctiveAssignmentInConstructor
Lint/EmptyConditionalBody
Lint/EmptyEnsure
Lint/LiteralInInterpolation
Lint/NonLocalExitFromIterator
Lint/ParenthesesAsGroupedExpression
Lint/RedundantCopDisableDirective
Lint/RedundantRequireStatement
Lint/RedundantSafeNavigation
Lint/RedundantStringCoercion
Lint/RedundantWithIndex
Lint/RedundantWithObject
Lint/SafeNavigationChain
Lint/SafeNavigationConsistency
Lint/SelfAssignment
Lint/UnreachableCode
Lint/UselessMethodDefinition
Lint/Void

Previous PRs:
Lint/ShadowedArgument
Lint/DuplicateMethods
Lint/BooleanSymbol
RSpec/SpecFilePathSuffix

script/bulk_import/base.rb

@@ -14,7 +14,6 @@ if ARGV.include?("bbcode-to-md")
 end
 
 require "pg"
-require "set"
 require "redcarpet"
 require "htmlentities"
 
@@ -29,8 +28,6 @@ class BulkImport::Base
   NOW ||= "now()"
   PRIVATE_OFFSET ||= 2**30
 
-  # rubocop:disable Layout/HashAlignment
-
   CHARSET_MAP = {
     "armscii8" => nil,
     "ascii" => Encoding::US_ASCII,
@@ -70,8 +67,6 @@ class BulkImport::Base
     "utf8" => Encoding::UTF_8,
   }
 
-  # rubocop:enable Layout/HashAlignment
-
   def initialize
     charset = ENV["DB_CHARSET"] || "utf8"
     db = ActiveRecord::Base.connection_db_config.configuration_hash

script/bulk_import/vbulletin.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require_relative "base"
-require "set"
 require "mysql2"
 require "htmlentities"
 require "parallel"

script/bulk_import/vbulletin5.rb

@@ -2,7 +2,6 @@
 
 require_relative "base"
 require "cgi"
-require "set"
 require "mysql2"
 require "htmlentities"
 require "ruby-bbcode-to-md"