Remove TLS version fallback code

[bgrainger]

In #458, code was added to try TlsVersions.None first, then fall back to TLS 1.1 if that failed. This was done to accommodate yaSSL-based servers (MySQL <= 5.7.28).

This has caused multiple problems over the years:

• yaSSL/Schannel TLS 1.2 workaround causing unintended TLS downgrade #1132
• Connection Pool permanently falls back to TLS 1.1 #1349
• Missing certificate causes The collection already contains item with same key 'net.transport' exception #1405
• System.InvalidOperationException: key 'net.transport' still ocours, #1376 (comment)

In addition, it makes debugging TLS connection issues harder:

• Authentication failed because the remote party sent a TLS alert: 'ProtocolVersion' #1407

Since this fix was only for a major version of MySQL Server that is no longer supported (5.7), there are minor versions that don't have the problem (>= 5.7.28), TLS 1.1 should generally be avoided, and there's now a Tls Versions connection string option that could be used as a workaround, the fallback code should simply be deleted and affected users advised to use the connection string setting.

[bgrainger]

It should also be safe to delete the fallback code added in 715c3b5 for #482.

[bgrainger]

Fixed in 2.3.4.