Drop obsolete compiler fallback in stdenv

This code originated in a slightly different form in 8e88b22fd1ec65a344ce6e4facd6dad4b415b2ad:

https://github.com/Homebrew/homebrew/blob/8e88b22fd1ec65a344ce6e4facd6dad4b415b2ad/Library/Homebrew/extend/ENV.rb#L30-L32

Back then, MacOS.default_compiler could return nil, which meant
ENV.compiler could do the same. This code was carried forward as the
surrounding code changed. At this point it should be unreachable.

Library/Homebrew/extend/ENV/std.rb

@@ -58,17 +58,7 @@ module Stdenv
 
     append 'LDFLAGS', '-Wl,-headerpad_max_install_names'
 
-    # set us up for the user's compiler choice
-    self.send self.compiler
-
-    # we must have a working compiler!
-    unless cc
-      @compiler = MacOS.default_compiler
-      self.send @compiler
-      self.cc  = MacOS.locate("cc")
-      self.cxx = MacOS.locate("c++")
-    end
-
+    send(compiler)
     validate_cc!(formula) unless formula.nil?
 
     if cc =~ GNU_GCC_REGEXP