Fix unused variable warnings in hwpmc_mod.c
With clang 15, the following -Werror warnings are produced:
sys/dev/hwpmc/hwpmc_mod.c:4805:6: error: variable 'nfree' set but not used [-Werror,-Wunused-but-set-variable]
int nfree;
^
sys/dev/hwpmc/hwpmc_mod.c:4804:6: error: variable 'ncallchains' set but not used [-Werror,-Wunused-but-set-variable]
int ncallchains;
^The 'nfree' and 'ncallchains' variables were used in KASSERTs, but these
were removed due to refactoring in d9f1b8dbf29d. Remove the variables
since they no longer serve any purpose.
MFC after: 3 days