utils: nuke bitfield functions and macros

utils: nuke bitfield functions and macros

The bitfield functions and macros were committed without explaining
their purpose in the commit message.

As they are only used in uci, and conflict with similar functions added
in hostapd, breaking our hostapd ubus patch, nuke them from libubox and
add them in uci instead.

If we need them anywhere else in the future we can add it to libubox
again, but preferably prefixed with ubox_.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>