Cache: removed dead store in ngx_http_file_cache_vary_header().

Found by Clang Static Analyzer.

src/http/ngx_http_file_cache.c

diff --git a/src/http/ngx_http_file_cache.c b/src/http/ngx_http_file_cache.c
index bc4c806..e7072e1 100644
--- a/src/http/ngx_http_file_cache.c
+++ b/src/http/ngx_http_file_cache.c
@@ -1022,7 +1022,6 @@
         /* normalize spaces */
 
         p = header[i].value.data;
-        start = p;
         last = p + header[i].value.len;
 
         while (p < last) {