Removed and renamed function

* src/rmatio.c (create_char_MAT_variable): Removed function

* src/rmatio.c (write_charsxp, write_strsxp): Replaced call to
  create_char_MAT_variable with call to Mat_VarCreate

* src/rmatio.c (create_empty_MAT_variable) Renamed to
  Mat_VarCreateEmpty, Changed return to matvar_t pointer. Return NULL
  on error.