[fidl][llcpp] Remove caller allocating methods from the main interface

Equivalent functionality have been added to
WireWeakAsyncBufferClientImpl and WireWeakOnewayBufferClientImpl, and
user code have been migrated to `.buffer(...)`.

Bug: 85688

Change-Id: If39a47832c4eacc0d587d22aa7ad539cf1eca068
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/626817
Reviewed-by: Benjamin Prosnitz <bprosnitz@google.com>
Commit-Queue: Yifei Teng <yifeit@google.com>
Fuchsia-Auto-Submit: Yifei Teng <yifeit@google.com>