env/ext4.c: remove CONFIG_CMD_SAVEENV ifdef

env/ext4.c: remove CONFIG_CMD_SAVEENV ifdef

Removing this ifdef/endif pair yields a "defined but unused warning"
for CONFIG_CMD_SAVEENV=n, but that vanishes if we use the ENV_SAVE_PTR
macro instead. This gives slightly better compile testing, and
moreover, it's possible to have

  CONFIG_CMD_SAVEENV=n
  CONFIG_SPL_SAVEENV=y
  SPL_ENV_IS_IN_EXT4=y

in which case env_ext4_save would erroneously not be compiled in.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>