[generator] Don't special case synthetic methods

[jonpryor]

Context: dotnet/android#1998 (comment)
Context: https://github.com/xamarin/monodroid/commit/2c515356d9a671f729aff2e81c4c2f6184749722

Long ago, generator was written to consume API XML descriptions
provided by Google. This approach worked reasonably well until
Honeycomb, at which point Google didn't provide an XML API description
when the API-H android.jar was released.

The fix was jar2xml, which used Java Reflection around android.jar
to generate a similar API XML description which generator could use.

jar2xml in turn had its own share of problems, which resulted in
class-parse, which had a different share of problems. :-)

The original hope was that generator could directly use
class-parse output. Turns out, it wasn't that easy, hence
Xamarin.Android.Tools.ApiXmlAdjuster, which took class-parse
output and "munged" it into something resembling jar2xml output
(e.g. with respect to whether method overrides were included).

However, before Xamarin.Android.Tools.ApiXmlAdjuster existed, I
tried to get generator to consume class-parse output directly, and
in that spirit xamarin/monodroid@2c515356 updated generator to skip
processing of synthetic methods:

Don't emit synthetic methods, as this results in "duplicate" methods
-- one for the "real" method, one for the synthetic method.

However, the scenario monodroid@2c515356 attempted to address --
direct support for class-parse output -- is not something that has
ever been used, or been tested, or even worked.

Thus, the special-casing of synthetic methods is effectively dead
code, all the more so because Xamarin.Android.Tools.ApiXmlAdjuster
never emitted the //method/@synthetic attribute in the first place,
so generators special-casing of synthetic methods was never hit.

...until 0881acc, which updated
Xamarin.Android.Tools.ApiXmlAdjuster to include and copy over all
source attributes, including //method/@synthetic.

NOW generator began to see methods with
//method[@synthetic='true'], and skip them.

The resulting code compiled, but had ABI breakage.

Remove the special-casing logic for synthetic methods from
generator. monodroid@2c515356 never supported direct consumption
of class-parse output by generator, meaning such an effort will
require more work anyway, and removing synthetic special-casing from
generator will allow it to consume post-0881accd
Xamarin.Android.Tools.ApiXmlAdjuster output without breaking things.

[jonpryor]

The reported unit test failure makes no sense at all, so I've restarted that build.

/me crosses fingers for the next build...

[jonpryor]

Rebuild is successful!

[atsushieno]

We'll have to keep using ApiXmlAdjuster for at least a few years because otherwise metadata fixups from anyone won't work as expected anymore.