Remove redundant compiler flag
`-Wunused' is included in `-Wall -Wextra'.
# To compile with PKCS11: add "-lpkcs11-helper" to LDFLAGS
CFLAGS ?= -O2
-WARNING_CFLAGS ?= -Wall -Wextra -Wunused
+WARNING_CFLAGS ?= -Wall -Wextra
LDFLAGS ?=
LOCAL_CFLAGS = $(WARNING_CFLAGS) -I../include -D_FILE_OFFSET_BITS=64
