drivers/radeon: remove unused CXXFLAGS, LLVM_CPP_FILES

drivers/radeon: remove unused CXXFLAGS, LLVM_CPP_FILES

The above two variables are unused as of commit

  commit 024fe6852a76f33d7e2afc5621340e387c381bb0
  Author: Tom Stellard <thomas.stellard@amd.com>
  Date:   Tue Apr 2 10:42:50 2013 -0700

      radeon/llvm: Use LLVM C API for compiling LLVM IR to ISA v2

which removed the only cpp file from drivers/radeon, but missed to
remove the CXXFLAGS. The sequential commit reintroduced and empty
LLVM_CPP_FILES.

Lets cleanup and remove both.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>