Remove BackgroundConnectionResetHelper (used by DeferConnectionReset=false)

[bgrainger]

BackgroundConnectionResetHelper (used when DeferConnectionReset = false, the default from 1.3.0 to 1.3.9 inclusive) appears to be the source of a number of bugs/deadlocks:

• Connection pool stuck - some (not all) connections stuck in Failed after failure  #990
• Updating from alpha.2 to any newer makes connection not closing PomeloFoundation/Pomelo.EntityFrameworkCore.MySql#1439
• Getting lots of exceptions from BackgroundConnectionResetHelper #993

Its initial implementation added a lot of extra latency: #982

It was disabled by default in c4d66c0.

Remove the code (and ignore the connection string option) until a more robust implementation can be developed.