ruby: Avoid environment flags when building gprc code

#27660 caused `CPPFLAGS` to inherit
from the environment, but this can cause the Makefile to use external
include files for re2 and other libraries if `-I` flags are defined.

This commit reverts to the original behavior of only using
`RbConfig::CONFIG` values to avoid using the wrong headers.

src/ruby/ext/grpc/extconf.rb

@@ -30,20 +30,11 @@
 
 ENV['MACOSX_DEPLOYMENT_TARGET'] = '10.10'
 
-def env_unset?(name)
-  ENV[name].nil? || ENV[name].size == 0
-end
-
-def rbconfig_set?(name)
-  RbConfig::CONFIG[name] && RbConfig::CONFIG[name].size > 0
-end
-
 def inherit_rbconfig(name)
-  ENV[name] = RbConfig::CONFIG[name] if env_unset?(name) && rbconfig_set?(name)
+  ENV[name] = RbConfig::CONFIG[name] || ''
 end
 
 def env_append(name, string)
-  ENV[name] ||= ''
   ENV[name] += ' ' + string
 end