Revert "Allow inclusion of entropy.h when it's disabled"

This reverts commit 424210a93c05450a7a8a5b0f6255d788b3f93f74.

This change was not safe enough for an LTS branch, as it might break code
that assumes it's safe to declare an object of type mbedtls_entropy_context
even when MBEDTLS_ENTROPY_C is undefined.
diff --git a/include/mbedtls/entropy.h b/include/mbedtls/entropy.h
index 8bf5d71..ca06dc3 100644
--- a/include/mbedtls/entropy.h
+++ b/include/mbedtls/entropy.h
@@ -86,8 +86,6 @@
 #define MBEDTLS_ENTROPY_SOURCE_STRONG   1       /**< Entropy source is strong   */
 #define MBEDTLS_ENTROPY_SOURCE_WEAK     0       /**< Entropy source is weak     */
 
-#if defined(MBEDTLS_ENTROPY_C)
-
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -288,6 +286,4 @@
 }
 #endif
 
-#endif /* MBEDTLS_ENTROPY_C */
-
 #endif /* entropy.h */