CLEANUP: proxy: Remove dead code when parsing "http-restrict-req-hdr-names" option

negation or default modifiers are not supported for this option. However,
this was already tested earlier in cfg_parse_listen() function. Thus, when
"http-restrict-req-hdr-names" option is parsed, the keyword modifier is
always equal to KWM_STD. It is useless to test it again at this place.

This patch should solve the issue #1702.
diff --git a/src/cfgparse-listen.c b/src/cfgparse-listen.c
index e7cd651..1bff2cd 100644
--- a/src/cfgparse-listen.c
+++ b/src/cfgparse-listen.c
@@ -2460,13 +2460,6 @@
 			} /* end while loop */
 		}
 		else if (strcmp(args[1], "http-restrict-req-hdr-names") == 0) {
-			if (kwm != KWM_STD) {
-				ha_alert("parsing [%s:%d]: negation/default is not supported for option '%s'.\n",
-					 file, linenum, args[1]);
-				err_code |= ERR_ALERT | ERR_FATAL;
-				goto out;
-			}
-
 			if (alertif_too_many_args(2, file, linenum, args, &err_code))
 				goto out;