removed the old Android app

Running git-annex linux builds in termux seems to work well enough that the
only reason to keep the Android app would be to support Android 4-5, which
the old Android app supported, and which I don't know if the termux method
works on (although I see no reason why it would not).
According to [1], Android 4-5 remains on around 29% of devices, down from
51% one year ago.

[1] https://www.statista.com/statistics/271774/share-of-android-platforms-on-mobile-devices-with-android-os/

This is a rather large commit, but mostly very straightfoward removal of
android ifdefs and patches and associated cruft.

Also, removed support for building with very old ghc < 8.0.1, and with
yesod < 1.4.3, and without concurrent-output, which were only being used
by the cross build.

Some documentation specific to the Android app (screenshots etc) needs
to be updated still.

This commit was sponsored by Brett Eisenberg on Patreon.

CmdLine/Action.hs

@@ -24,10 +24,7 @@ import Control.Concurrent.STM
 import Control.Exception (throwIO)
 import Data.Either
 import qualified Data.Map.Strict as M
-
-#ifdef WITH_CONCURRENTOUTPUT
 import qualified System.Console.Regions as Regions
-#endif
 
 {- Runs a command, starting with the check stage, and then
  - the seek stage. Finishes by running the continutation, and 
@@ -169,7 +166,6 @@ callCommandActionQuiet = start
 
 {- Do concurrent output when that has been requested. -}
 allowConcurrentOutput :: Annex a -> Annex a
-#ifdef WITH_CONCURRENTOUTPUT
 allowConcurrentOutput a = do
 	fromcmdline <- Annex.getState Annex.concurrency
 	fromgitcfg <- annexJobs <$> Annex.getGitConfig
@@ -197,9 +193,6 @@ allowConcurrentOutput a = do
 		setconcurrentoutputenabled False
 	setconcurrentoutputenabled b = Annex.changeState $ \s ->
 		s { Annex.output = (Annex.output s) { concurrentOutputEnabled = b } }
-#else
-allowConcurrentOutput = id
-#endif
 
 {- Ensures that only one thread processes a key at a time.
  - Other threads will block until it's done. -}