logging: don't clamp the log level to DFATAL in debug builds

logging: don't clamp the log level to DFATAL in debug builds

Summary:
Remove the logic that clamps the log level to DFATAL in debug builds.

This behavior logically makes some sense, but results in subtle and potentially
confusing behavior changes.  It seems confusing that after calling
`setLevel(LogLevel::MAX_LEVEL)` on a log category, calling `getLevel()` on that
object would return `DFATAL` rather than `MAX_LEVEL`.  We also weren't clamping
the level to DFATAL consistently: `setLevel()` would clamp the value, but on
construction the initial level was still set to MAX_LEVEL rather than DFATAL.
This resulted in some issues when implementing `LoggerDB::getConfig()` since we
could not consistently detect if a log category was using the default level
settings.  Rather than fix this inconsistency it seems better to simply remove
this clamping behavior.

This means that it is possible for users to disable DFATAL log messages even in
debug builds if they really want to.  Previously this was only allowed in
release builds.  This protection doesn't really seem all that
valuable--presumably most developers won't really want to do this, and if they
really do request this configuration it doesn't seem all that bad to honor it.

Reviewed By: bolinfest, yfeldblum

Differential Revision: D6200569

fbshipit-source-id: 83ef8e2e4d3b61bc5b105038cbe3132979e9ac67