Fix unused variable warnings in tcp_hpts.c

With clang 15, the following -Werror warning is produced:

sys/netinet/tcp_hpts.c:1114:10: error: variable 'paced_cnt' set but not used [-Werror,-Wunused-but-set-variable]
        int32_t paced_cnt = 0;
                ^
sys/netinet/tcp_hpts.c:1112:11: error: variable 'total_slots_processed' set but not used [-Werror,-Wunused-but-set-variable]
        uint64_t total_slots_processed = 0;
                 ^

The 'paced_cnt' variable was in tcp_hpts.c when it was first added, and
the 'total_slots_processed' variable was added in d7955cc0ffdf9, but
both appear to have been debugging aids that have never been used, so
remove them.

MFC after: 3 days

(cherry picked from commit b33bfe6e157429ad764e9a883a5e3a194a6d1f9f)