Hide "bootstrap class path not set in conjunction with -source" messages

javac warns whenever -source is used with a version that does not
match javac, because it wants an rt.jar provided for the given
source level.  We are not using -source to prevent usage of APIs
that don't exist in the newer rt.jar, so hide the messages.

Test: javac_wrapper_test.go
Change-Id: Ic94b61767436848fc164e377dc3cc5a63726f1cf