Remove unneeded code

This code was triggering the following warnings:

  src/mathed/InsetMathHull.cpp:234:15: error: variable 'counters_to_save' is not needed and will not be emitted [-Werror,-Wunneeded-internal-declaration]
          const char * counters_to_save[] = {"section", "chapter"};
                       ^
  src/mathed/InsetMathHull.cpp:235:21: error: unused variable 'numcnts' [-Werror,-Wunused-const-variable]
          unsigned int const numcnts = sizeof(counters_to_save)/sizeof(char *);

The original code was put in at 645f9fdf (and the comment at 8a226ec4).

This commit amends a65e12d6.