Strict DER signatures are always enforced; remove the flag and code that used it.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

src/script/interpreter.h

@@ -45,7 +45,8 @@ enum
     SCRIPT_VERIFY_STRICTENC = (1U << 1),
 
     // Passing a non-strict-DER signature to a checksig operation causes script failure (softfork safe, BIP62 rule 1)
-    SCRIPT_VERIFY_DERSIG    = (1U << 2),
+    // In Zcash this is required, and validation of non-strict-DER signatures is not implemented.
+    //SCRIPT_VERIFY_DERSIG    = (1U << 2),
 
     // Passing a non-strict-DER signature or one with S > order/2 to a checksig operation causes script failure
     // (softfork safe, BIP62 rule 5).