Remove remaining reference to MIXED_ADDITION macro

[dtebbs]

USE_MIXED_ADDITION was used in several contexts, controlling both the assumed form of base elements in multiexp, AND and some cases whether or not to batch-convert intermediate values to special form.

#45 allows the input base elements to be specified as a (template) parameter, but the macro is still used in the BDL012 implementation of multiexp. This would be better handled as some kind of parameter (a new method BDLO12_... or another template parameter, which could apply in several situations, such a BDLO12_signed).

With this, it should be possible to remove the macro and CMake parameter.