Clean up old version guards in preparation for the upcoming OpenSSL 3.0
support.

OpenSSL 1.0.1 reached its EOL on 2016-12-31. At that time, we decided
to keep 1.0.1 support because many major Linux distributions were still
shipped with 1.0.1. Now, nearly 4 years later, most Linux distributions
are reaching their EOL and it should be safe to assume nobody uses them
anymore. Major ones that were using 1.0.1:

 - Ubuntu 14.04 is EOL since 2019-04-30
 - RHEL 6 will reach EOL on 2020-11-30

LibreSSL 3.0 and older versions are no longer supported by the LibreSSL
team as of October 2020.

Note that OpenSSL 1.0.2 also reached EOL on 2019-12-31 and 1.1.0 also
did on 2018-08-31.

https://github.com/ruby/openssl/commit/c055938f4b