asn1: Remove dependency on X.509

Doxygen will fail to build if we have references to files that don't
exist. Since we are planning on removing X.509 soon, we even need to
remove explicit Doxygen references to X.509 things as those will no
longer resolve once the X.509 files are deleted.

fixup! asn1: Remove dependency on X.509
diff --git a/include/mbedtls/asn1.h b/include/mbedtls/asn1.h
index 96c1c9a..f80acd7 100644
--- a/include/mbedtls/asn1.h
+++ b/include/mbedtls/asn1.h
@@ -66,7 +66,7 @@
  * - 0x02 -- tag indicating INTEGER
  * - 0x01 -- length in octets
  * - 0x05 -- value
- * Such sequences are typically read into \c ::mbedtls_x509_buf.
+ * Such sequences are typically read into Mbed TLS's \c mbedtls_x509_buf.
  * \{
  */
 #define MBEDTLS_ASN1_BOOLEAN                 0x01