libquota: remove quota_is_on() which was the last user of quotactl()

The quotactl() system call was being used without the use of a
function prototype.  On closer examination, it turns out the one user
of that system call was the quota_is_on() function, which is not used
by e2fsprogs at all.  Since libquota is an e2fsprogs-internal library,
and not one that we plan to export any time soon, the simplest thing
to do is to simply remove quota_is_on(), which in turn allows us to
remove all of the infrastructure around using the Linux-specific
quotactl() system call.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/configure b/configure
index ae599cd..b741112 100755
--- a/configure
+++ b/configure
@@ -10917,7 +10917,7 @@
 fi
 
 fi
-for ac_func in  	__secure_getenv 	backtrace 	blkid_probe_get_topology 	chflags 	fallocate 	fallocate64 	fchown 	fdatasync 	fstat64 	ftruncate64 	getdtablesize 	getmntinfo 	getpwuid_r 	getrlimit 	getrusage 	jrand48 	llseek 	lseek64 	mallinfo 	mbstowcs 	memalign 	mmap 	msync 	nanosleep 	open64 	pathconf 	posix_fadvise 	posix_memalign 	prctl 	quotactl 	setmntent 	setresgid 	setresuid 	srandom 	strcasecmp 	strdup 	strnlen 	strptime 	strtoull 	sync_file_range 	sysconf 	usleep 	utime 	valloc
+for ac_func in  	__secure_getenv 	backtrace 	blkid_probe_get_topology 	chflags 	fallocate 	fallocate64 	fchown 	fdatasync 	fstat64 	ftruncate64 	getdtablesize 	getmntinfo 	getpwuid_r 	getrlimit 	getrusage 	jrand48 	llseek 	lseek64 	mallinfo 	mbstowcs 	memalign 	mmap 	msync 	nanosleep 	open64 	pathconf 	posix_fadvise 	posix_memalign 	prctl 	setmntent 	setresgid 	setresuid 	srandom 	strcasecmp 	strdup 	strnlen 	strptime 	strtoull 	sync_file_range 	sysconf 	usleep 	utime 	valloc
 do :
   as_ac_var=`$as_echo "ac_cv_func_$ac_func" | $as_tr_sh`
 ac_fn_c_check_func "$LINENO" "$ac_func" "$as_ac_var"