Remove async-await pair

[jnyrup]

As GetOrAddAsync only calls the other GetOrAddAsync method with an extra null parameter it should not be necessary to await the call.
This also makes up for the a missing ConfigureAwait(false) introduced in 218bd23#diff-4a28b127c5a26901c14d4513be185990R165

[svengeance]

Method	Mean	Error	StdDev	Ratio	Gen 0	Gen 1	Gen 2	Allocated
DotNetMemoryCache_GetOrAddAsync	372.7 ns	11.15 ns	0.61 ns	1.00	0.0334	-	-	280 B
LazyCache_GetOrAddAsync_Proposed	516.4 ns	16.93 ns	0.93 ns	1.39	0.0448	-	-	376 B
LazyCache_GetOrAddAsync	584.4 ns	5.17 ns	0.28 ns	1.57	0.0534	-	-	448 B

Here's some numbers with the proposed change as well. Good find!

[jnyrup]

@svengeance Thanks for providing the numbers that confirms the change.

If you're a heap allocation hunter, you might be interested in ClrHeapAllocationAnalyzer