mm/: remove reference to non-existent readme

remove reference to non-existent readme in mm/

Pointing to the documentation page doesn't make sense in this case,
because it doesn't explain the use of `#undef XXX` for this case anyway.

Signed-off-by: raiden00pl <raiden00@railab.me>

mm/umm_heap/umm_calloc.c

@@ -45,7 +45,7 @@
  *
  ****************************************************************************/
 
-#undef calloc /* See mm/README.txt */
+#undef calloc
 FAR void *calloc(size_t n, size_t elem_size)
 {
 #if defined(CONFIG_ARCH_ADDRENV) && defined(CONFIG_BUILD_KERNEL)

mm/umm_heap/umm_free.c

@@ -45,7 +45,7 @@
  *
  ****************************************************************************/
 
-#undef free /* See mm/README.txt */
+#undef free
 void free(FAR void *mem)
 {
   mm_free(USR_HEAP, mem);

mm/umm_heap/umm_malloc.c

@@ -51,7 +51,7 @@
  *
  ****************************************************************************/
 
-#undef malloc /* See mm/README.txt */
+#undef malloc
 FAR void *malloc(size_t size)
 {
 #if defined(CONFIG_ARCH_ADDRENV) && defined(CONFIG_BUILD_KERNEL)

mm/umm_heap/umm_malloc_size.c

@@ -36,7 +36,7 @@
  * Public Functions
  ****************************************************************************/
 
-#undef malloc_size /* See mm/README.txt */
+#undef malloc_size
 size_t malloc_size(FAR void *mem)
 {
   return mm_malloc_size(USR_HEAP, mem);

mm/umm_heap/umm_memalign.c

@@ -50,7 +50,7 @@
  *
  ****************************************************************************/
 
-#undef memalign /* See mm/README.txt */
+#undef memalign
 FAR void *memalign(size_t alignment, size_t size)
 {
 #if defined(CONFIG_ARCH_ADDRENV) && defined(CONFIG_BUILD_KERNEL)

mm/umm_heap/umm_realloc.c

@@ -52,7 +52,7 @@
  *
  ****************************************************************************/
 
-#undef realloc /* See mm/README.txt */
+#undef realloc
 FAR void *realloc(FAR void *oldmem, size_t size)
 {
 #if defined(CONFIG_ARCH_ADDRENV) && defined(CONFIG_BUILD_KERNEL)

mm/umm_heap/umm_zalloc.c

@@ -51,7 +51,7 @@
  *
  ****************************************************************************/
 
-#undef zalloc /* See mm/README.txt */
+#undef zalloc
 FAR void *zalloc(size_t size)
 {
 #ifdef CONFIG_ARCH_ADDRENV