ssl_poll_perf: address compiler warnings

[esyr]

The changes address the following compiler warnings (reported by GCC 14):

ssl_poll_perf.c: In function ‘srvapp_setup_response’:
ssl_poll_perf.c:1553:33: warning: variable ‘pesb’ set but not used [-Wunused-but-set-variable]
 1553 |     struct poll_event_sbstream *pesb;
      |                                 ^~~~
ssl_poll_perf.c: In function ‘clntapp_write_custreamcb’:
ssl_poll_perf.c:2389:28: warning: unused variable ‘rb’ [-Wunused-variable]
 2389 |     struct request_buffer *rb;
      |                            ^~
ssl_poll_perf.c: In function ‘clntapp_update_pec’:
ssl_poll_perf.c:2469:9: warning: variable ‘e’ set but not used [-Wunused-but-set-variable]
 2469 |     int e;
      |         ^
ssl_poll_perf.c: In function ‘main’:
ssl_poll_perf.c:3138:9: warning: unused variable ‘ccount’ [-Wunused-variable]
 3138 |     int ccount = 0;
      |         ^~~~~~
ssl_poll_perf.c: At top level:
ssl_poll_perf.c:1007:1: warning: ‘stream_state_str’ defined but not used [-Wunused-function]
 1007 | stream_state_str(int stream_state)
      | ^~~~~~~~~~~~~~~~
ssl_poll_perf.c:954:1: warning: ‘err_str_n’ defined but not used [-Wunused-function]
  954 | err_str_n(unsigned long e, char *buf, size_t buf_sz)
      | ^~~~~~~~~