entropy: fix warning reported by llvm

/Users/steve/repos/x265/source/encoder/entropy.cpp:321:44: warning: equality comparison with extraneous parentheses[-Wparentheses-equality]
    if ((cu->getSlice()->m_numRefIdx[list] == 1))
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
slice: remove m_bLMvdL1Zero

This feature, which signals L1 MVD is zero for an entire slice, was never
enabled because we never use the same frame list in L0 and L1.
dpb: remove check for B frame without L1

our lookahead won't emit B frames without a leading P frame
dpb: simplify check for m_bCheckLDC; make note for future optimization
entropy: slice->m_colFromL0Flag is a bool
encoder: do some slice initializations only once
slice: do not initialize fields that are always written unconditionally
slice: make xGetRefPic a PicList method
slice: remove getWpScaling()
slice: remove unused initWpScaling()