Remove some now-unused code from mbfl_strcut

The legacy mbfl_strcut function is only used to implement mb_strcut
for legacy text encodings which 1) do not use a fixed number of bytes
per codepoint, 2) do not have an 'mblen_table' which can be used to
quickly determine the codepoint length of a byte sequence, and 3) do
not have a specialized 'mb_cut' function which implements mb_strcut
for that text encoding.

Remove unused code from mbfl_strcut, and leave only what is currently
needed for the implementation of mb_strcut.

Author: alexdowad