lib/rsa: Remove support for OpenSSL < 1.1.0 and libressl < 2.7.0

lib/rsa: Remove support for OpenSSL < 1.1.0 and libressl < 2.7.0

Older OpenSSL and libressl versions have a slightly different API.
This require #ifdefs to support. However, we still can't support it
because the ECDSA path does not compile with these older versions.
These #ifdefs are truly a vestigial appendage.

Alternatively, the ECDSA path could be updated for older libraries,
but this requires significant extra code, and #ifdefs. Those libraries
are over three years old, and there concerns whether it makes sense to
build modern software for real world use against such old libraries.

Thusly, remove #ifdefs and code for old OpenSSL and LibreSSL support.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>