[lldb] Remove some unnecessary scoped timers in TypeSystemSwiftTypeRef

[kastiglione]

In Instruments traces, these two timers show to be somewhat high firing, while also having quite short durations.

Name	Count	Duration	Min Duration	Avg Duration	Std Dev Duration	Max Duration
GetCanonicalDemangleTree	4,214	224.15 ms	9.24 µs	53.19 µs	61.87 µs	620.08 µs
GetDemangledType	3,849	141.02 ms	8.51 µs	36.64 µs	13.83 µs	142.81 µs

These don't seem to bring any value, and do add noise and overhead to the trace. For that reason, I think they can be deleted. See also #3637.

(cherry picked from #3743)