Remove mbedtls_low_level_sterr() and mbedtls_high_level_strerr()

Just removed from the API. We can greatly simplify error.c but that will be
for later.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/error-unification.txt b/ChangeLog.d/error-unification.txt
index e1790d2..a19e60c 100644
--- a/ChangeLog.d/error-unification.txt
+++ b/ChangeLog.d/error-unification.txt
@@ -5,3 +5,7 @@
      Mbed TLS error codes..
      This will not affect most applications since in both cases, the
      error values are between -32767 and -1 as before.
+
+Removals
+   * Remove mbedtls_low_level_sterr() and mbedtls_high_level_strerr(),
+     since these concepts no longer exists. There is just mbedtls_strerror().