net: lib: coap: Remove unused function

Building with clang warns:

subsys/net/lib/coap/coap_link_format.c:39:20: error: unused function
'append_be16' [-Werror,-Wunused-function]
static inline bool append_be16(struct coap_packet *cpkt, uint16_t data)

Signed-off-by: Tom Hughes <tomhughes@chromium.org>