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.
diff --git a/tests/data_files/Makefile b/tests/data_files/Makefile
index e75bf81..99d64eb 100644
--- a/tests/data_files/Makefile
+++ b/tests/data_files/Makefile
@@ -869,14 +869,6 @@
 	$(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
 ################################################################
diff --git a/tests/data_files/secp521r1_prv.der b/tests/data_files/secp521r1_prv.der
deleted file mode 100644
index 4d342bd..0000000
--- a/tests/data_files/secp521r1_prv.der
+++ /dev/null
Binary files differ