CLEANUP: proxy: simplify proxy_parse_rate_limit proxy checks

rate-limits are valid for both frontend and listen, but not backend; so
we can simplify this check in a similar manner as it is done in e.g
max-keep-alive-queue.

this should fix github issue #449

Signed-off-by: William Dauchy <w.dauchy@criteo.com>
diff --git a/src/proxy.c b/src/proxy.c
index 8720b28..aed32f9 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -330,7 +330,7 @@
                                   struct proxy *defpx, const char *file, int line,
                                   char **err)
 {
-	int retval, cap;
+	int retval;
 	char *res;
 	unsigned int *tv = NULL;
 	unsigned int *td = NULL;
@@ -341,7 +341,6 @@
 	if (strcmp(args[1], "sessions") == 0) {
 		tv = &proxy->fe_sps_lim;
 		td = &defpx->fe_sps_lim;
-		cap = PR_CAP_FE;
 	}
 	else {
 		memprintf(err, "'%s' only supports 'sessions' (got '%s')", args[0], args[1]);
@@ -359,10 +358,9 @@
 		return -1;
 	}
 
-	if (!(proxy->cap & cap)) {
-		memprintf(err, "%s %s will be ignored because %s '%s' has no %s capability",
-			 args[0], args[1], proxy_type_str(proxy), proxy->id,
-			 (cap & PR_CAP_BE) ? "backend" : "frontend");
+	if (!(proxy->cap & PR_CAP_FE)) {
+		memprintf(err, "%s %s will be ignored because %s '%s' has no frontend capability",
+			  args[0], args[1], proxy_type_str(proxy), proxy->id);
 		retval = 1;
 	}
 	else if (defpx && *tv != *td) {