build: Remove check for `openssl/ec.h`

We don't use any elliptic curves from OpenSSL anymore, nor include this
header anywhere but optionally in the tests of secp256k1 (which has
its own autoconf setup).

Reported by sinetek on IRC.

configure.ac

@@ -691,14 +691,6 @@ fi
 
 LIBZCASH_LIBS="-lgmp -lgmpxx -lboost_system-mt -lcrypto -lsodium $RUST_LIBS"
 
-CXXFLAGS_TEMP="$CXXFLAGS"
-LIBS_TEMP="$LIBS"
-CXXFLAGS="$CXXFLAGS $SSL_CFLAGS $CRYPTO_CFLAGS"
-LIBS="$LIBS $SSL_LIBS $CRYPTO_LIBS $GMP_LIBS $GMPXX_LIBS"
-AC_CHECK_HEADER([openssl/ec.h],, AC_MSG_ERROR(OpenSSL ec header missing),)
-CXXFLAGS="$CXXFLAGS_TEMP"
-LIBS="$LIBS_TEMP"
-
 AC_MSG_CHECKING([whether to build bitcoind])
 AM_CONDITIONAL([BUILD_BITCOIND], [test x$build_bitcoind = xyes])
 AC_MSG_RESULT($build_bitcoind)