[0.1.x] chore: remove old async-await support (#1742)

## Motivation

Currently, the tests for `tokio` 0.1's async-await support build against a 
fairly old nightly from Rust 1.36. Upstream changes to a transitive 
dependency introduced a use of `MaybeUninit`, which is feature 
flagged on this nightly. This resultedin [0.1.x builds breaking][1].

The `tokio` 0.1 async-await support has not been maintained, in favour
of working on 0.2. It currently uses severely outdated versions of the
async-await APIs (including the `await!` macro). Anyone using
async-await with Tokio is almsot certainly on 0.2 by now.

## Solution

Since the 0.1 async-await APIs are both unused and unmaintained, this
branch deletes them.

[1]: https://dev.azure.com/tokio-rs/Tokio/_build/results?buildId=3174&view=logs&jobId=ba363064-0d45-526e-6c63-c7e816804fbe&taskId=3aff0ee6-e312-56d4-f5d3-d804e6c343c3&lineStart=83&lineEnd=87&colStart=1&colEnd=1

Signed-off-by: Eliza Weisman <[email protected]>

azure-pipelines.yml

@@ -68,13 +68,6 @@ jobs:
       tokio-buf:
         - util
 
-# Run async-await tests
-- template: ci/azure-test-nightly.yml
-  parameters:
-    name: test_nightly
-    displayName: Test Async / Await
-    rust: nightly-2019-04-25
-
 # Try cross compiling
 - template: ci/azure-cross-compile.yml
   parameters:
@@ -105,7 +98,6 @@ jobs:
       - test_sub_cross
       - test_linux
       - features
-      - test_nightly
       - cross_32bit_linux
       - minrust
       - tsan