Remove duplicate include statement

Now that the Error module has error codes as well and is processed by
the generate_errors script like any other module, we don't need to
include the header manually.
diff --git a/library/error.c b/library/error.c
index 53b3a26..c451f4d 100644
--- a/library/error.c
+++ b/library/error.c
@@ -25,8 +25,7 @@
 #include MBEDTLS_CONFIG_FILE
 #endif
 
-#if defined(MBEDTLS_ERROR_C) || defined(MBEDTLS_ERROR_STRERROR_DUMMY)
-#include "mbedtls/error.h"
+#if defined(MBEDTLS_ERROR_STRERROR_DUMMY)
 #include <string.h>
 #endif