Remove deprecated --use-{gcc,llvm,clang}

Closes Homebrew/homebrew#29380.

Library/Homebrew/cmd/install.rb

@@ -6,19 +6,6 @@ module Homebrew extend self
   def install
     raise FormulaUnspecifiedError if ARGV.named.empty?
 
-    {
-      'gcc' => 'gcc-4.2',
-      'llvm' => 'llvm-gcc',
-      'clang' => 'clang'
-    }.each_pair do |old, new|
-      opt = "--use-#{old}"
-      if ARGV.include? opt then opoo <<-EOS.undent
-        #{opt.inspect} is deprecated and will be removed in a future version.
-        Please use "--cc=#{new}" instead.
-        EOS
-      end
-    end
-
     if ARGV.include? '--head'
       raise "Specify `--HEAD` in uppercase to build from trunk."
     end

Library/Homebrew/extend/ENV/shared.rb

@@ -91,16 +91,6 @@ module SharedEnvExtension
           raise "Invalid value for --cc: #{other}"
         end
       end
-    elsif ARGV.include? '--use-gcc'
-      if MacOS.locate("gcc-4.2") || HOMEBREW_PREFIX.join("opt/apple-gcc42/bin/gcc-4.2").exist?
-        :gcc
-      else
-        raise "gcc-4.2 not found!"
-      end
-    elsif ARGV.include? '--use-llvm'
-      :llvm
-    elsif ARGV.include? '--use-clang'
-      :clang
     elsif homebrew_cc
       cc = COMPILER_ALIASES.fetch(homebrew_cc, homebrew_cc)
       COMPILER_SYMBOL_MAP.fetch(cc) { MacOS.default_compiler }