Remove macro that does not belong in 2.x

MBEDTLS_ALLOW_PRIVATE_ACCESS existed in development and was copied
over whilst cherry-picking commits.

Signed-off-by: Joe Subbiani <joe.subbiani@arm.com>
diff --git a/library/common.h b/library/common.h
index 61254fa..c064724 100644
--- a/library/common.h
+++ b/library/common.h
@@ -52,13 +52,6 @@
 #define MBEDTLS_STATIC_TESTABLE static
 #endif
 
-/** Allow library to access its structs' private members.
- *
- * Although structs defined in header files are publicly available,
- * their members are private and should not be accessed by the user.
- */
-#define MBEDTLS_ALLOW_PRIVATE_ACCESS
-
 /** Byte Reading Macros
  *
  * Given a multi-byte integer \p x, MBEDTLS_BYTE_n retrieves the n-th