MDEV-22929 fixup: root_name() clash with clang++ <fstream>

The clang++ -stdlib=libc++ header file <fstream> depends on
<filesystem> that defines a member function path::root_name(),
which conflicts with the rather unused #define root_name()
that had been introduced in
commit 7c58e97b.

Because an instrumented -stdlib=libc++ (rather than the default
-stdlib=libstdc++) is easier to build for a working -fsanitize=memory
(cmake -DWITH_MSAN=ON), let us remove the conflicting #define for now.

include/my_sys.h

@@ -873,7 +873,6 @@ extern void my_free_lock(void *ptr);
 #define my_malloc_lock(A,B) my_malloc(PSI_INSTRUMENT_ME, (A),(B))
 #define my_free_lock(A) my_free((A))
 #endif
-#define root_name(A) ""
 #define alloc_root_inited(A) ((A)->min_malloc != 0)
 #define ALLOC_ROOT_MIN_BLOCK_SIZE (MALLOC_OVERHEAD + sizeof(USED_MEM) + 8)
 #define clear_alloc_root(A) do { (A)->free= (A)->used= (A)->pre_alloc= 0; (A)->min_malloc=0;} while(0)

mysys/my_alloc.c

@@ -174,7 +174,7 @@ void *alloc_root(MEM_ROOT *mem_root, size_t length)
 #if defined(HAVE_valgrind) && defined(EXTRA_DEBUG)
   reg1 USED_MEM *next;
   DBUG_ENTER("alloc_root");
-  DBUG_PRINT("enter",("root: %p  name: %s", mem_root, root_name(mem_root)));
+  DBUG_PRINT("enter",("root: %p", mem_root));
 
   DBUG_ASSERT(alloc_root_inited(mem_root));
 
@@ -208,7 +208,7 @@ void *alloc_root(MEM_ROOT *mem_root, size_t length)
   reg2 USED_MEM **prev;
   size_t original_length __attribute__((unused)) = length;
   DBUG_ENTER("alloc_root");
-  DBUG_PRINT("enter",("root: %p  name: %s", mem_root, root_name(mem_root)));
+  DBUG_PRINT("enter",("root: %p", mem_root));
   DBUG_ASSERT(alloc_root_inited(mem_root));
 
   DBUG_EXECUTE_IF("simulate_out_of_memory",
@@ -387,8 +387,7 @@ void free_root(MEM_ROOT *root, myf MyFlags)
 {
   reg1 USED_MEM *next,*old;
   DBUG_ENTER("free_root");
-  DBUG_PRINT("enter",("root: %p  name: %s  flags: %u", root, root_name(root),
-                      (uint) MyFlags));
+  DBUG_PRINT("enter",("root: %p  flags: %lu", root, MyFlags));
 
 #if !(defined(HAVE_valgrind) && defined(EXTRA_DEBUG))
   /*