BUG/BUILD: debug: fix unused variable error

BUG/BUILD: debug: fix unused variable error

A compilation error occurs when using DEBUG_MEM_STATS due to a variable
now being unused in debug_iohandler_memstats() :

src/debug.c: In function ‘debug_iohandler_memstats’:
src/debug.c:1862:24: error: unused variable ‘sc’ [-Werror=unused-variable]
 1862 |         struct stconn *sc = appctx_sc(appctx);
      |                        ^~

This is caused since the following commit :
  94b8ed446f70a381dde0ea2dc891b62fcc3cc8e1
  MEDIUM: cli/applet: Stop to test opposite SC in I/O handler of CLI commands

This must not be backported.