env: mmc: cosmetic: remove unused macro STR(X)

Remove the unused macro STR(X) since the commit 2b2f727500dc ("env: mmc:
allow support of mmc_get_env_dev with OF_CONTROL")

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
diff --git a/env/mmc.c b/env/mmc.c
index 8941e0f..8576141 100644
--- a/env/mmc.c
+++ b/env/mmc.c
@@ -21,9 +21,6 @@
 #include <errno.h>
 #include <dm/ofnode.h>
 
-#define __STR(X) #X
-#define STR(X) __STR(X)
-
 #define ENV_MMC_INVALID_OFFSET ((s64)-1)
 
 #if defined(CONFIG_ENV_MMC_USE_DT)