Drop @OPENSSL_LIB@ in bigkey

@OPENSSL_LIB@ was brought back with the 2f9f6f1f revert.

https://gitlab.isc.org/isc-projects/bind9/-/jobs/1288815:

$ & "C:/Strawberry/perl/bin/perl.exe" Configure "with-tools-version=15.0" "with-platform-toolset=v141" "with-platform-version=10.0.17763.0" "with-vcredist=C:/Program Files (x86)/Microsoft Visual Studio/2017/BuildTools/VC/Redist/MSVC/14.16.27012/vcredist_x64.exe" "with-openssl=C:/OpenSSL" "with-libxml2=C:/libxml2" "with-libuv=C:/libuv" "without-python" "with-system-tests" x64
unknown control @OPENSSL_LIB@ in ../bin/tests/system/win32/bigkey.vcxproj

See #1728 (closed)