Update ChangeLog

Remove extra entries added by a bad cherry-pick.
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 = mbed TLS x.x.x branch released xxxx-xx-xx
 
 Bugfix
-   * Fix the key_app_writer example which was writing a leading zero byte which
-     was creating an invalid ASN.1 tag. Found by Aryeh R. Fixes #1257.
-   * Fix compilation error on C++, because of a variable named new.
-     Found and fixed by Hirotaka Niisato in #1783.
-   * Fix "no symbols" warning issued by ranlib when building on Mac OS X. Fix
-     contributed by tabascoeye in pull request #1600.
-   * Clarify documentation for mbedtls_ssl_write() to include 0 as a valid
-     return value. Found by @davidwu2000. #839
-   * Fix a memory leak in mbedtls_x509_csr_parse(), found by catenacyber,
-     Philippe Antoine. Fixes #1623.
    * Correct the documentation for `mbedtls_ssl_get_session()`.
      This API has deep copy of the session, and the peer
      certificate is not lost. Fixes #926.