src: remove flagcmp expression

src: remove flagcmp expression

This expression is not used anymore, since:

 ("src: transform flag match expression to binop expression from parser")

remove it.

This completes the revert of c3d57114f119 ("parser_bison: add shortcut
syntax for matching flags without binary operations"), except the parser
chunk for backwards compatibility.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>