Remove ASN1_STRING_length_set prototype.

This was removed in
https://boringssl-review.googlesource.com/c/boringssl/+/42724 but I
missed the prototype.

Change-Id: I345f0e9ea114c35cbf35d2b97b228bfa1ba9e188
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/43764
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/asn1.h

@@ -646,7 +646,6 @@ OPENSSL_EXPORT int 		ASN1_STRING_cmp(const ASN1_STRING *a, const ASN1_STRING *b)
 OPENSSL_EXPORT int 		ASN1_STRING_set(ASN1_STRING *str, const void *data, int len);
 OPENSSL_EXPORT void		ASN1_STRING_set0(ASN1_STRING *str, void *data, int len);
 OPENSSL_EXPORT int ASN1_STRING_length(const ASN1_STRING *x);
-OPENSSL_EXPORT void ASN1_STRING_length_set(ASN1_STRING *x, int n);
 OPENSSL_EXPORT int ASN1_STRING_type(const ASN1_STRING *x);
 OPENSSL_EXPORT unsigned char * ASN1_STRING_data(ASN1_STRING *x);
 OPENSSL_EXPORT const unsigned char *ASN1_STRING_get0_data(const ASN1_STRING *x);