BUILD: fd: remove unused variable totlen in fd_write_frag_line()

Ilya reports in GH #1392 that clang 13 complains about totlen being
calculated and not used in fd_write_frag_line(), which is true. It's
a leftover of some older code.
diff --git a/src/fd.c b/src/fd.c
index f746dff..88fa3b7 100644
--- a/src/fd.c
+++ b/src/fd.c
@@ -545,7 +545,6 @@
 ssize_t fd_write_frag_line(int fd, size_t maxlen, const struct ist pfx[], size_t npfx, const struct ist msg[], size_t nmsg, int nl)
 {
 	struct iovec iovec[32];
-	size_t totlen = 0;
 	size_t sent = 0;
 	int vec = 0;
 	int attempts = 0;
@@ -572,7 +571,6 @@
 		iovec[vec].iov_base = pfx->ptr;
 		iovec[vec].iov_len  = MIN(maxlen, pfx->len);
 		maxlen -= iovec[vec].iov_len;
-		totlen += iovec[vec].iov_len;
 		if (iovec[vec].iov_len)
 			vec++;
 		pfx++; npfx--;