Remove CC & CFLAGS from MemorySanitizer example

They are now unnecessary for projects written in Rust, since
backtrace-rs used by the standard library has only Rust dependencies.

Author: tmiasko

src/doc/unstable-book/src/compiler-flags/sanitizer.md

@@ -196,10 +196,6 @@ fn main() {
 
 ```shell
 $ export \
-  CC=clang \
-  CXX=clang++ \
-  CFLAGS='-fsanitize=memory -fsanitize-memory-track-origins' \
-  CXXFLAGS='-fsanitize=memory -fsanitize-memory-track-origins' \
   RUSTFLAGS='-Zsanitizer=memory -Zsanitizer-memory-track-origins' \
   RUSTDOCFLAGS='-Zsanitizer=memory -Zsanitizer-memory-track-origins'
 $ cargo clean