Fix/unused includes

[dforsi]

This PR deletes some includes that the tool iwyu says that are unused.
It seems safe to remove them because make doesn't show new warnings and make check still succeeds:

PASS: testcpp.sh
============================================================================
Testsuite summary for Hamlib 4.5~git
============================================================================
# TOTAL: 1
# PASS:  1
# SKIP:  0
# XFAIL: 0
# FAIL:  0
# XPASS: 0
# ERROR: 0
============================================================================
[...]
PASS: testrig.sh
PASS: testfreq.sh
PASS: testbcd.sh
PASS: testloc.sh
PASS: testrigcaps.sh
PASS: testcache.sh
PASS: testcookie.sh
PASS: testgrid.sh
============================================================================
Testsuite summary for Hamlib 4.5~git
============================================================================
# TOTAL: 8
# PASS:  8
# SKIP:  0
# XFAIL: 0
# FAIL:  0
# XPASS: 0
# ERROR: 0
============================================================================

There are more includes that could be deleted or moved from a *.h to the corresponding *.c, but these other changes are better done in a future PR:

- #include <arpa/inet.h>
- #include <cal.h>
- #include <config.h>
- #include <ctype.h>
- #include <errno.h>
- #include <fcntl.h>
- #include <getopt.h>
- #include <hamlib/amplifier.h>
- #include <hamlib/config.h>
- #include <hamlib/rig.h>
- #include <hamlib/rotator.h>
- #include <iofunc.h>
- #include <limits.h>
- #include <linux/types.h>
- #include <misc.h>
- #include <netinet/in.h>
- #include <network.h>
- #include <pthread.h>
- #include <register.h>
- #include <serial.h>
- #include <signal.h>
- #include <stdarg.h>
- #include <stdbool.h>
- #include <stdint.h>
- #include <stdlib.h>
- #include <sys/ioctl.h>
- #include <sys/param.h>
- #include <sys/socket.h>
- #include <sys/stat.h>
- #include <sys/time.h>
- #include <sys/types.h>
- #include <token.h>
- #include <tones.h>
- #include <unistd.h>