Remove unused test data file

Since "Remove component designed to test MAX_SIGNATURE_SIZE",
secp521r1_prv.der is no longer used.

ec_521_prv.pem can be used for the same purpose.

tests/data_files/Makefile

@@ -869,14 +869,6 @@ ec_prv.pk8param.pem: ec_prv.pk8param.der
 	$(OPENSSL) pkey -in $< -inform DER -out $@
 all_final += ec_prv.pk8param.pem
 
-###
-### A generic SECP521R1 private key
-###
-
-secp521r1_prv.der:
-	$(OPENSSL) ecparam -genkey -name secp521r1 -noout -out secp521r1_prv.der
-all_final += secp521r1_prv.der
-
 ################################################################
 ### Generate CSRs for X.509 write test suite
 ################################################################