Fix build (broken by removal of key_arg from SSL_SESSION parsing).

This fixes error reported by clang:
unused variable 'kKeyArgTag' [-Werror,-Wunused-const-variable].

Change-Id: I1d5c9937064bfadd810cbe1b73e0070cc2ead684
Signed-off-by: Piotr Sikora <piotr@cloudflare.com>
Reviewed-on: https://boringssl-review.googlesource.com/2070
Reviewed-by: Adam Langley <agl@google.com>

ssl/ssl_asn1.c

@@ -121,7 +121,6 @@
  * Note: When the relevant features were #ifdef'd out, support for
  * parsing compressionMethod [11] and srpUsername [12] was lost. */
 
-static const int kKeyArgTag = CBS_ASN1_CONTEXT_SPECIFIC | 0;
 static const int kTimeTag =
     CBS_ASN1_CONSTRUCTED | CBS_ASN1_CONTEXT_SPECIFIC | 1;
 static const int kTimeoutTag =