xdiff: remove unused structure, functions, and constants
`bdiffparam_t` is unused. `xdl_fall_back_diff` is no longer used after
D2573. `XDL_MMB_READONLY`, `XDL_MMF_ATOMIC` are unused. `XDL_BDOP*` are
unused since there is no xdiff binary diff algorithm. `anchors` feature is
not used. It's also relatively new in git.
Differential Revision: https://phab.mercurial-scm.org/D2684