Refine the behaviour of -dno-debug-output and -dtrace-level

It wasn't clear to me what these flags should do and what they should turn off.

From the user guide.

`-dno-debug-output`	Suppress any unsolicited debugging output. When GHC has been built with the DEBUG option it occasionally emits debug output of interest to developers. The extra output can confuse the testing framework and cause bogus test failures, so this flag is provided to turn it off.
`-dtrace-level`	Not present

-dno-debug-output currently suppresses the output of -ddump-tc-trace which is certainly not "unsolicited". It should be used, and is, in Outputable to suppress the output of pprTrace.

dtrace-level is used in a few places but always as if was a boolean flag. I don't really see what the point of it is. The only commit which mentions it is "8cdb98b9" and I can't find the origin of the flag. I think we should just remove it.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Task
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture