Remove @conditionalClientFlatten and config arm-resource-flattening

[pshao25]

Here is some background for these two sets of decorators for flattening purpose.

1. @conditionalClientFlatten and its config arm-resource-flattening in Azure.ResourceManager
2. @flattenProperty in TCGC

Previously,

1. Swagger emitter only emits @conditionalClientFlatten. It doesn't emit @flattenProperty because @flattenProperty is only allowed in client.tsp
2. TCGC emits @flattenProperty and @conditionalClientFlatten.

Therefore we need both of them.

Now,

1. According to Two resources with one properties flattens while the other not #1931, @flattenProperty is agreed to be put into main.tsp, so swagger emitter can emit it. See this.
2. TCGC emits @flattenProperty.

Therefore @flattenProperty suffices for both scenarios.

Considering these two decorators are so confusing that we always take several hours/days to make people understand, and even ourselves cannot understand it easily, it seems we could remove @conditionalClientFlatten and its config arm-resource-flattening to improve the usability.

[markcowl]

Since @conditionalClientFlatten is private, it should not be used in specs, so agreed that specs should only use @flattenProperty

[pshao25]

Talked to @allenjzhang , we will remove the @conditionalClientFlatten and its config arm-resource-flattening. For the impacted service, we will use @@flattenProperty to mitigate.

[allenjzhang]

This will be a lower pri cleanup items post 1.0

[lmazuel]

Guidance is to never use conditionalClientFlatten, and always use flattenProperty.

TypeSpec team will be free to clean their code at their convenience, once we confirmed to them no spec are still using conditionalClientFlatten