We used to have the pattern of including log messages at the beginning
of `Execute()` in MSBuild tasks:
    Log.LogDebugMessage ("MyMSBuildTask Task");
    Log.LogDebugMessage ("  FooProperty: {0}", FooProperty);
    Log.LogDebugTaskItems ("  BarItems: ", BarItems);
This was mostly here to improve diagnostic log output from XBuild,
which we no longer support.
MSBuild properly logs any input properties on MSBuild tasks, so we can
remove a lot of code!
There were a couple places I could go a step further:
* `<AndroidCreateDebugKey/>` and `<AndroidZipAlign/>` I could remove
  the entire `Execute()` method.
* `<JavaCompileToolTask/>` had an `OnLogStarted` method I could
  remove. `<Javac/>` also was overriding this method.
I thought this might have *some* impact on build times, since logging
and `string.Format` takes some time (it's not free!).
A build of the Xamarin.Forms app in this repo:
    # Before
    Time Elapsed 00:00:25.66
    Time Elapsed 00:00:25.36
    Time Elapsed 00:00:25.67
    # After
    Time Elapsed 00:00:24.58
    Time Elapsed 00:00:23.98
    Time Elapsed 00:00:24.98
The overall build times vary quite a bit. So if I took a conservative
estimate it seems the initial build is 500ms better?
A more revealing difference, I think, is the size of these binlog
files:
    # fresh build
    746947 before.first.binlog
    740797 after.first.binlog
    # build with no changes
    827913 before.second.binlog
    825665 after.second.binlog
    # build with xaml change
    848821 before.third.binlog
    844365 after.third.binlog
These files are compressed, so if they are 2K-6K smaller that was a
lot of log messages...