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

Remove the unused macro STR(X) since the commit 2b2f7275 ("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>