RSA PSS: remove redundant check; changelog

Remove a check introduced in the previous buffer overflow fix with keys of
size 8N+1 which the subsequent fix for buffer start calculations made
redundant.

Added a changelog entry for the buffer start calculation fix.
diff --git a/ChangeLog b/ChangeLog
index 6f7637d..4a2e710 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -18,6 +18,8 @@
    * Fix memory leak in mbedtls_ssl_set_hostname() when called multiple times.
      Found by projectgus and jethrogb, #836.
    * Fix usage help in ssl_server2 example. Found and fixed by Bei Lin.
+   * Fix some invalid RSA-PSS signatures with keys of size 8N+1 that were
+     accepted. Generating these signatures required the private key.
 
 = mbed TLS 2.6.0 branch released 2017-08-10