package/libopenssl: drop useless option for rc5

Commit a83d4186 (package/libopenssl: add option to enable some
features) added an option to enable rc5. However, since commit
1fff9412 (Fixup non-x86 openssl build), dated 2002-12-30, rc5
has always been forcibly disabled in Buildroot.

Given that it was unconditionally disabled all this time, and no
one complained, it means there is virtually no-one using rc5, so we
can just drop the option.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: "GAUTRON, Erwan" <erwan.gautron@bertin.fr>
Cc: "Weber, Matthew L Collins" <Matthew.Weber@collins.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>