env/sf.c: drop private CMD_SAVEENV logic

env/sf.c: drop private CMD_SAVEENV logic

Deciding whether to compile the env_sf_save() function based solely on
CONFIG_SPL_BUILD is wrong: For U-Boot proper, it leads to a build
warning in case CONFIG_CMD_SAVEENV=n (because the env_save_ptr() macro
causes the function to indeed not be referenced anywhere). And for
SPL, when one selects CONFIG_SPL_SAVEENV, one obviously expects to
actually be able to save the environment.

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