build: Drop redundant `sys/sysctl.h` header check
The AC_CHECK_HEADERS macro defines HAVE_SYS_SYSCTL_H if the sys/sysctl.h header is found. However, in the source code, this header is guarded by HAVE_SYSCTL and HAVE_SYSCTL_ARND macros, which have their own checks. Since HAVE_SYS_SYSCTL_H is not used, we can skip the AC_CHECK_HEADERS(... sys/sysctl.h ...) check.