[CT-2587] DelayedFileHandler is Obsolete, Should Be Removed in 1.5 and Later Versions

[peterallenwebb]

The DelayedFileHandler is a class in Core's legacy Logbook logging system which captured log lines fired before log file initialization and buffered them to be written when the log file was created. Since version 1.5.0, Core no longer fires events between these two points in time. Furthermore, because the new Click-based API no longer makes a separate call to set a file path for the legacy logger, the buffer is never flushed and sometimes overflows when legacy logging is used (e.g. by dbt-cloud).

Since DelayedFileHandler no longer serves a useful purpose, and it is causing expensive errors, we will simply remove it in 1.5.1rc2+.

[peterallenwebb]

Change in main is done, I am now preparing the backport to 1.5.latest, which @ChenyuLInx has suggested we test with dbt-rpc to ensure no breaking changes.