[Xamarin.Android.Tools.AndroidSdk] Revert JDK validation

[jonpryor]

Context: dotnet/android#2004
Context: https://jenkins.mono-project.com/job/xamarin-android-pr-builder/3656/testReport/junit/Xamarin.Android.Build.Tests/ResolveSdksTaskTests/ResolveSdkTiming___Debug/

Before commit 7553056, the only validation performed on the
AndroidSdkInfo's javaSdkPath parameter was "does
{javaSdkPath}/bin/jarsigner exist?

This is the world the xamarin-android unit tests assumed, as it
would create {javaSdkPath}/bin/jarsigner and
{javaSdkPath}/bin/javac.bash (among others), but not
{javaSdkPath/bin/javac.

Then came commit 7553056, which altered
AndroidSdkUnix.ValidateJavaSdkLocation() semantics so that it now
required the presence of {javaSdkPath}/bin/javac.

This changed caused the xamarin-android unit tests to fail:

JavaSdkPath should be /Users/builder/jenkins/workspace/xamarin-android-pr-builder/xamarin-android/bin/TestDebug/temp/ResolveSdkTiming/jdk
Expected string length 63 but was 115. Strings differ at index 1.
Expected: "/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home"
But was:  "/Users/builder/jenkins/workspace/xamarin-android-pr-builder/x..."
------------^

The "expected" and "actual" values are reversed; the actual problem is
that the javaSdkPath value provided to the AndroidSdkInfo
constructor --
/Users/builder/jenkins/workspace/xamarin-android-pr-builder/xamarin-android/bin/TestDebug/temp/ResolveSdkTiming/jdk
-- is "ignored", because it doesn't pass validation,
prompting AndroidSdkInfo to call AndroidSdkUnix.GetJavaSdkPath(),
which returns a "system" location,
/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home.

As such, the original javaSdkPath value is not preserved and
returned by the AndroidSdkInfo.JavaSdkPath property.

Fix this by removing the AndroidSdkUnix.ValidateJavaSdkLocation()
method: it doesn't appear to actually be useful in this instance, as
the base AndroidSdkBase.ValidateJavaSdkLocation() already checks
for the presence of {javaSdkPath}/bin/jarsigner.