MINOR: config: remove support for deprecated option "tune.chksize"

It was marked as deprecated for immediate removal as it was not used,
let's reject it and remove it from the doc. A specific error suggests
to check tune.bufsize instead.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 5107890..55469de 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -1055,7 +1055,6 @@
    - tune.buffers.limit
    - tune.buffers.reserve
    - tune.bufsize
-   - tune.chksize
    - tune.comp.maxlevel
    - tune.fd.edge-triggered
    - tune.h2.header-table-size
@@ -2431,9 +2430,6 @@
   value set using this parameter will automatically be rounded up to the next
   multiple of 8 on 32-bit machines and 16 on 64-bit machines.
 
-tune.chksize <number> (deprecated)
-  This option is deprecated and ignored.
-
 tune.comp.maxlevel <number>
   Sets the maximum compression level. The compression level affects CPU
   usage during compression. This value affects CPU usage during compression.
diff --git a/src/cfgparse-global.c b/src/cfgparse-global.c
index b539a97..84e4406 100644
--- a/src/cfgparse-global.c
+++ b/src/cfgparse-global.c
@@ -32,7 +32,7 @@
 	"insecure-fork-wanted", "insecure-setuid-wanted", "nosplice",
 	"nogetaddrinfo", "noreuseport", "quiet", "zero-warning",
 	"tune.runqueue-depth", "tune.maxpollevents", "tune.maxaccept",
-	"tune.chksize", "tune.recv_enough", "tune.buffers.limit",
+	"tune.recv_enough", "tune.buffers.limit",
 	"tune.buffers.reserve", "tune.bufsize", "tune.maxrewrite",
 	"tune.idletimer", "tune.rcvbuf.client", "tune.rcvbuf.server",
 	"tune.sndbuf.client", "tune.sndbuf.server", "tune.pipesize",
@@ -224,8 +224,9 @@
 		global.tune.maxaccept = max;
 	}
 	else if (strcmp(args[0], "tune.chksize") == 0) {
-		ha_warning("parsing [%s:%d]: the option '%s' is deprecated and will be removed in next version.\n",
-			   file, linenum, args[0]);
+		ha_alert("parsing [%s:%d]: option '%s' is not supported any more (tune.bufsize is used instead).\n", file, linenum, args[0]);
+		err_code |= ERR_ALERT | ERR_FATAL;
+		goto out;
 	}
 	else if (strcmp(args[0], "tune.recv_enough") == 0) {
 		if (alertif_too_many_args(1, file, linenum, args, &err_code))