configure.ac: drop unneeded dependency check

configure.ac: drop unneeded dependency check

The script configure.ac checks for the presence of system include
files required to build OpenOCD.

It incorrectly check for:
	dirent.h
	pthread.h
	ifaddrs.h
	net/if.h
that are never included by OpenOCD source code, plus the generated
macros:
	HAVE_DIRENT_H
	HAVE_PTHREAD_H
	HAVE_IFADDRS_H
	HAVE_NET_IF_H
are never used in OpenOCD source code.

It also checks for the system function:
	vasprintf()
that is never called by OpenOCD source code, plus the generated
macro:
	HAVE_VASPRINTF
is never used in OpenOCD source code.

Drop the checks for the unused system include files and the unused
system function.

Change-Id: I68c1b1a1be268a830247fc489f210877c32d7d23
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7175
Tested-by: jenkins