sed: Remove -Werror=format-nonliteral from compiler flags

sed uses non-literal format strings in a few places.
This has been verified to be harmless in 4.8.

Removing -Werror=format-nonliteral here allows us to use
-Werror=format-nonliteral globally in OPTIMIZE_FOR=security mode
while keeping sed building.

TARGET_CFLAGS_remove = "-Werror=format-nonliteral"

Signed-off-by: Bernhard Rosenkränzer <bernhard.rosenkraenzer.ext@huawei.com>