Remove outdated exercise instructions for gdb.

After refactoring the code, it doesn't produce any warnings, so it's
pointless to check for any.

Author: hageboeck

code/debug/README.md

@@ -5,7 +5,6 @@
 * inspect backtrace, variables
 * find problem and fix bug
 * try stepping, breakpoints
-* use -Wall -Wextra and see warning
 
 
 ### Go back to the "valgrind" exercise

talk/tools/debugging.tex

@@ -64,7 +64,6 @@
     \item inspect backtrace, variables
     \item find problem and fix bug
     \item try stepping, breakpoints
-    \item use -Wall -Wextra and see warning
     \end{itemize}
   \end{exercise}
 \end{frame}