Update mscorlib.xml to match mono/2017-12/82306753 (#15)

Context: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/940/API_Compatibility_Checks/
Context: dotnet/android@90805bd
Context: dotnet/android#1492

xamarin-android bumped to mono/2017-12/82306753, which resulted in
many reported API removals:

	<h3>Type Changed: System.Memory`1</h3>
	<p>Removed method:</p>
	<pre>
	  <span class='removed removed-method breaking' data-is-breaking>public Buffers.MemoryHandle Retain (bool);</span>
	</pre>

	<h3>Type Changed: System.ReadOnlyMemory`1</h3>
	<p>Removed method:</p>
	<pre>
	  <span class='removed removed-method breaking' data-is-breaking>public Buffers.MemoryHandle Retain (bool);</span>
	</pre>

	<h3>Removed Type <span class='breaking' data-is-breaking>System.MemoryExtensions</span></h3>
	...

This breakage [is expected][0]:

[0]: https://gitter.im/xamarin/xamarin-android?at=5ac398ee270d7d37088b2da2

> the api diff is expected, we had to pull some API out as corefx
> folks didn't finish it on time (the types were new in dev15-7)

Update `reference/mscorlib.xml` so that these removed members no
longer report any errors.

Author: atsushieno