common: remove b64_decode

We don't use it, and it's buggy (will always return NULL)

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

common/base64.h

@@ -5,6 +5,5 @@
 #include <ccan/tal/tal.h>
 
 char *b64_encode(const tal_t *ctx, const u8 *data, size_t len);
-u8 *b64_decode(const tal_t *ctx, const char *str, size_t len);
 
 #endif /* LIGHTNING_COMMON_BASE64_H */